Okay, I swapped my R9700Pro for a good old, trusty Voodoo3 3500

Test setup :
AthlonXP 2000+
256MB DDR
Voodoo3 3500/TV
Win98SE

Driver settings :
Video Quality : High
Alpha Blending : Sharper
Resolution : 800x600 16bit

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At first, I tested various Win9x Drivers to see what works best.
To my surprise, I did not find any real difference between the 3dfx V1.07.00WHQL, V1.07.00beta and NuDriver5. They might differ a bit in speed, but did not show any difference in image quality, or fix any of the bugs, which are shown below.

I also did a test with the famous V1.03.00 DX7 Driver, that worked so well at least for intel CPU owners with the Game GTA3.
Well, there was no difference, except an additional, significant error which causes the entire Floor to "disappear" all around the Player, for almost half of the entire viewing distance
So that way, you're walking 'open air' with no Floor visible and that in the end renders the Game completely unplayable of course...

So here are my experiences on the Voodoo3 under Win98SE with the UT2003 Retail Game :

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Pros :

It's really still playable, and framerates are mostly a factor of the CPU, rather than the Voodoo3, if setup correctly.
What is needed is to set the Texture Detail low enough so the Framerate is not hit by exceeding the 16MB of the Card. It's easy to find out, since the Framecounter (hit 'tilde' Key to open Console, and enter "stat fps") does still calculate correctly even when in the Detail Settings Menu.

So just play around with anywhere from "Lowest" to "Normal" settings to get the Framerate you want.
Beware though, that "Normal" may work in Levels like DM-Antalus, but will slow to a crawl on larger maps like CTF-Face3 ! Detail setting "Lower" should yield the best compromise between Performance and Image quality.

This of course goes hand in hand with :
a) the number of Bots and
b) the Texture Level for the Player Models (Normal or Lower recommended)

After that, you should be seeing normal framerates; on the test machine (AthlonXP 2000+, 256MB DDR) those vary between sub-20 to 60; mostly averaging ~30 on the Voodoo3 3500.

Detail Textures (surprisingly) DO work flawless with the Voodoo3. The greenish discolorings I found on Voodoo4/5 when viewing at a Detail Textured Object (remember not all have Detail Textures) from a distance is not there, everything looks fine.
And good news is, that it doesn't take too big of a performance hit, but uprates the affected Textures quite a bit in quality

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Cons :

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Right from the Menu, you'll see that the Screen (Text, background Image) is looking quite ugly...
It seems that this is the normal consequence of the Game having to decompress the Textures and resize them all to 256x256 where needed.

Unfortunately, here comes one of the biggest drawbacks of UT2003 on a Voodoo3 :
Without large Texture support, all bigger Textures (when used) have to be resized...
That however, is not the biggest factor, as a Voodoo3 User will probably run the Game on fairly low Texture Detail anyway.

The biggest hit in Image Quality apparently comes from the lack of DXT (Texture compression)

The colors really look bland and lack vivid colors; there is also an error in Texture brightness coming from that, Stone Textures will look too dark, and lighter Sand or Ice Textures too bright in comparison.

The big Sky Textures used on open Maps really take the biggest hit, some rather look like 256 Color variants now

Same applies to the Coronas around light Sources found on lamps, or weapon effects. These really look more like 256 Colors in some cases

Problem is, that 16bit now really looks alot worse than what Voodoo3 Users are used to.
Driver setting will NOT help the Problem, as it is coming from within the Game itself , and cannot be corrected by Driver Advanced settings.

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There are still flashing Sky textures (DM-Antalus), just like we see on Voodoo4/5 under Win9x... This might not be the case with some Win2k/WinXP drivers, but I can't test or confirm that.

Shadows (from Player Characters and Objects) are non-existent, so one should leave the "Shadows" setting Disabled...

Projectors don't work as well, but are crash prone when enabled.
You really should disable those; the Benchmark (which is using Max. Detail by default) I could not get to run without a hard freeze

There are quite a few Z-Buffer inaccuracies, mostly when viewing at the Fractal Terrain from a distance, when it meets other Objects...
(Zig-Zag lines instead of smooth transition from Objects with the Floor)

Grass (Foilage) isn't displayed correctly, there are dark boxes around the Grass Textures all the Time, with all Drivers I tested

Some water surfaces carry a nasty Rainbox Box pattern.
Again, the same with all Drivers and Detail settings I tried
(Good thing is, there aren't too many Ponds around most levels, so this gross error won't kill off gameplay)