The game looks good, but id should be able to come up with something really more spectacular in 4 years.

Low Quality: This mode is designed for a graphics card with 64MB of Video RAM. It uses compressed textures (textures are the 2D images used on the surfaces of all 3D objects) and light maps, and texture resolution is also limited to 512x512 at most. The reason for this is that large uncompressed textures quickly fill up your Video RAM and cause a lot of swapping into and out of the Video RAM, creating small pauses and freezes. Texture compression and reducing the resolution of textures keeps the Video RAM overhead low, with the down side being some "compression artifacts" (i.e. blurriness, some pixelation and most noticeably a purplish tinge to textures). The game will also use only one sound per event. For example, firing the shotgun has the exact same sound effect every time. This is done to reduce memory usage, as all sounds are preloaded into RAM wherever possible.

Medium Quality: This mode is designed for a graphics card with 128MB of Video RAM. There is still compression used for textures, and light and render maps. Texture sizes are not automatically reduced however, so all textures will appear at their full resolution. There will still be compression artifacts however, but once again this must be done to keep Video RAM demand low. The number of sounds per event also increase, such that there will be some variety for each sound effect at the expense of some system RAM. This mode is the one recommended for most people as it is a good balance of image quality and performance.

High Quality: This mode is designed for a graphics card with 256MB of Video RAM. In this mode there is no texture compression (which greatly increases Video RAM usage), however light maps are compressed still. It is important to note that in this mode, Anisotropic Filtering is automatically enabled and set to 8x, which is quite high. This helps noticeably sharpen textures as they fade into the distance, however it has a pronounced impact on performance. Essentially in High Quality mode texture quality is improved over Medium Quality due to removal of compression and through use of Anisotropic Filtering, but at the cost of a significant performance hit on cards with lower Video RAM and processing power.

Ultra Quality: This mode is the highest possible for Doom 3 and is designed for a graphics card with 512MB (!) of Video RAM. No compression is used for anything, hence while graphics quality is at the best it can possibly be (not including Antialiasing, which you can also enable separately), there is a major performance hit for current systems. Due to the lack of compression loadup times are theoretically reduced, however in exchange for this most systems will notice a lot more freezes and pauses as video information is constantly being swapped into and out of your Video RAM. For the most part the difference between High and Ultra Quality modes is difficult to tell.

The Doom 3 Console
The console is the closest you can get to having direct real-time access to Doom 3's engine. Through the console you can enter commands to change engine variables during the game. To open the console, start Doom 3 and then press the CTRL, ALT and ~ keys together. You will see the console come down, and you can enter text at the command prompt. A list of commands you can enter are covered in the next section. To make regular access to the console easier, enter the following line into the console:
set com_allowconsole 1
This means that you can now open and close the console by simply pressing the '~' key (the key above TAB), and not three keys at once. Note that to make this setting (and many other settings) "stick", i.e. remain enabled even after quitting and restarting Doom 3, you will need to insert it into a Config file, or in the game icon's Target line, both of which are covered below.

Doom 3 Config Files
Although you can alter many variables by entering them into the console, most command variables need to be set each and every time you start Doom 3. This is because the game engine resets these variables to their default state as the engine initializes at startup. By default, when the game engine starts, it automatically detects the presence of, and runs the command variables from two specific files: DoomConfig.cfg and AutoExec.cfg. DoomConfig.cfg already exists in your \Doom 3\base\ directory, and you can edit it using a text editor like Windows WordPad or Notepad. However changing some of the settings in DoomConfig.cfg will see them reset to default values again the next time you restart Doom 3. So the best thing to do is create a new text file, rename it to "AutoExec.cfg" (without quotes) and place it in your \Doom 3\base\ directory. You can now enter all your custom tweaks and settings into this file, and they will come into effect automatically each and every time you start Doom 3. This makes it easier to keep track of what you have tweaked, and if at any time you want to remove all your tweaks, you can simply delete the AutoExec.cfg file.
Note that there is another way of executing a config file, and that is to firstly create a config file with any name you wish, e.g. MyTweaks.cfg, place it in the \Doom 3\base\ directory, and then in the Doom 3 console at any time type:
exec MyTweaks.cfg
This will run all the command variables in your new .cfg file, however unless you actually execute this file it will not be automatically run at startup time - only DoomConfig.cfg and AutoExec.cfg are automatically detected and run by Doom 3.


Performance Commands
[Cache Tweaks]
The following commands control the image cache, which if enabled and set correctly can help smooth FPS and may also boost performance:

image_useCache [0,1] - If set to 1, uses background loading to cache image information. This may not necessarily improve framerate performance, but it does assist in smoothing out frame rates and reducing loading pauses. Note that the image_cacheMinK (see below) size must be raised from its default otherwise the game will crash when using this setting. Once this setting is enabled, the actual cache value is set in the image_cacheMegs setting below.

image_cacheMegs [Megabytes] - Determines the maximum amount of system memory to allocate to temporary loading of full-sized precompressed images if the cache is enabled by setting image_useCache to 1. Note that since this caching only applies to precompressed images, using the Ultra Quality setting will render this setting useless, since at Ultra quality no texture compression is used. Since a cache is only a temporary holding area, and not the place where the entire game is meant to reside, do not raise this value to something extremely high as that will simply reduce the available memory for the rest of Doom 3. Try a value of 128MB for those with 512MB of system RAM, and if you have more than 1GB of RAM, you can try a higher value like 196MB or 256MB for example. Do not set this to half your system RAM or some other monstrous amount.

image_cacheMinK [KiloBytes] - This setting determines the minimum size in KB for precompressed image files to be loaded into the cache. To make sure most image files can be loaded, and to also prevent a system crash when enabling caching, select a value like 3072 to start with. Raising this setting may improve caching, but it may also hinder caching if most textures are below the size you specify, so don't just raise this value for the hell of it.

[Graphics Tweaks]

r_brightness [value] - Determines the overall brightness for the game. You can adjust this using the slider in the main options menu (See In-Game settings), however if you want to fine tune it you can do so here. Has no performance impact.

r_gamma [value] - Determines the gamma level (essentially a form of brightness) for the game. Raising this value too high can make the game image appear washed out. Has no performance impact.

r_displayRefresh [Hz] - Determines the refresh rate used when Doom 3 is launched. You should already be getting the maximum refresh rate possible on your monitor when playing Doom 3 through the use of a refresh rate fix (See my ATI Catalyst Tweak Guide for a link and instructions for both Nvidia and ATI users), however if you want to set this manually, determine the supported maximum refresh rate on your monitor for the resolution you've chosen for Doom 3 (e.g. I run Doom 3 at 1280x1024, and my monitor can do 85Hz maximum at this resolution), and set this variable to that value.

r_multiSamples [0,2,4,8,16] - This setting determines the level of Antialiasing samples used. This can be set using the in-game menus (See In-Game Settings section). The numerical values here are 0 which is no antialiasing, 2, 4, 8 and 16.

com_videoRam [MB] - Shows the detected amount of video memory on your graphics card in MB. If this value is incorrect, set it to the correct value to improve performance.

cm_backFaceCull [0,1] - If set to 1, removes all polygons behind the direction the player is facing - that is, all the things the player cannot see. This can improve performance, but if you experience any visual anomalies set this back to 0. Note it's cm_backFaceCull, not com_backFaceCull.

com_purgeAll [0,1] - If set to 1, discards all data between level loads. Can increase loading times noticeably but may help reduce in-game pauses.

com_machineSpec [-1,0,1,2,3] - Shows the currently detected machine specification level, with the choices being -1 - Not detected, 0 - Low quality, 1 - Medium quality, 2 - High quality, and 3 - Ultra quality.

com_fixedTic [-1,0,1] - If set to 0 (the default) Doom 3's ticrate is capped at 60FPS, meaning the game framerate cannot exceed this, except when running a timedemo. If you want to remove this cap, set com_fixedTic to -1. Setting it to 1 removes the cap, but causes the game physics to go out of sync. Removing the FPS cap doesn't necessarily improve performance, and can lead to more variable framerates.

g_projectileLights [0,1] - If set to 1, allows dynamic lights for projectiles. Setting this to 0 can improve performance in combat.

g_doubleVision [0,1] - If set to 1, shows a blurry "double vision" effect when taking damage. Setting this to 0 removes this effect and can improve performance when taking damage.

g_muzzleFlash [0,1] - If set to 1, shows a flash from the muzzle of weapons that are firing. If set to 0, this effect is disabled, which can improve performance.

g_showBrass [0,1] - If set to 1, visible shells will be ejected from weapons. If set to 0, no visible shells will be ejected, and performance may be improved in heavy combat.

g_showHud [0,1] - If set to 1, this enables the Heads Up Display (HUD). If set to 0, this removes the HUD from view, which may give a minor performance improvement. This command is most useful for taking "clean" screenshots (i.e. without the HUD).

g_showPlayerShadow [0,1] - If set to 1, the player will cast a shadow when standing near lights. If set to 0, the player casts no shadow, which improves performance.

g_nightmare [0,1] - If set to 1, Nightmare difficulty level can now be selected on the main menu screen. If set to 0, this option is disabled. Normally players must complete the entire game before Nightmare mode is enabled.

g_bloodEffects [0,1] - If set to 1, full blood effects (blood sprays, splattering etc.) is enabled. If set to 0, this is removed, which may yield a small performance improvement in heavy combat.

g_skipViewEffects [0,1] - If set to 1, removes various view-related special effects like the damage effects (double vision). This can improve performance.

g_fov [angle] - Specifies the angle for the Field of View (FOV). The wider the angle, the larger the field of vision for the player, giving a "fish eye" view in effect, however performance will be reduced. The smaller the angle, the higher the performance. The default of 90 is recommended, but players can try 85 which is a value used in some other games and will gain a slight boost in FPS.

g_skipFX [0,1] - If set to 1, disables certain special effects (I'm unclear on which). This can improve performance.

g_skipParticles [0,1] - If set to 1, disables certain particle effects (does not include smoke, dust, fog effects etc). This can improve performance, particularly in areas which use these effects to a large extent, but realism will be reduced.

image_usePrecompressedTextures [0,1] - When set to 1, all textures will be precompressed, resulting in less video memory being used and hence smoother performance on most machines. There will be some minor compression artifacts on textures, but these are hardly noticeable. If you want to use uncompressed textures for maximum image quality, set this to 0, but performance will suffer and you may get more small pauses depending on the amount of Video RAM on your graphics card.

image_useNormalCompression [1,2] - Specifies the compression method used for textures. 1 is 256 color compression (if available), which gives the best performance but provides the most compression artifacts, while 2 is rxgb compression, which is the normal compression method.

image_useCompression [0,1] - When set to 1, compression is used - with the method of compression specified in the image_useNormalCompression variable. Compression provides the fastest performance by reducing the size of textures and light maps, but there are some (minor) compression artifacts. When set to 0, no compression is used, which provides the highest image quality but slower performance.

image_preload [0,1] - If set to 1, Doom 3 attempts to preload most images in the background, meaning less loading pauses during the game but longer loading times. If set to 0, images are loaded dynamically as needed, meaning more loading pauses during the game. 1 is the recommended setting.

image_lodbias [value] - Determines the Level of Detail (LOD) bias for mipmapped images. The higher the number, the less detail is visible on surrounding objects. This can improve performance, but reduces image quality. The default is 0, and values above 1 reduce detail quite heavily, so experiment with fractional values such as 0.2 or 0.5.

image_anisotropy [1,2,4,8] - This setting determines the level of Anisotropic Filtering used in Doom 3. Valid values are 1, 2, 4, and 8. The higher the value, the lower your performance, but the crisper textures will appear in the distance. Note that 1 is the lowest you can set for Anisotropic - 0 will simply default back to 1. Also note that if you have forced Anisotropic Filtering in your graphics card's control panel, it will override this setting, so for optimal performance set the Anisotropic Filtering level in your control panel to 'Application Preference', and adjust the level here.

image_filter [GL_LINEAR_MIPMAP_NEAREST, GL_LINEAR_MIPMAP_LINEAR] - Determines the OpenGL texture filtering method to use on textures by default. GL_LINEAR_MIPMAP_LINEAR is Trilinear Filtering, however if you want a performance boost without any noticeable image quality loss, change this setting to GL_LINEAR_MIPMAP_NEAREST which is Bilinear Filtering.