Well, the actual Problem you noted right in your first Post :
If the Voodoo2 does not show up in the device Manager (Multimedia Device),
it is not properly installed.
Don't bother trying 3D Applications before that.
If your Windows System did not detect it at first boot after installation, recheck the Bios Screen at initial Boot-Up.
Depending on Bios, you should (for a very short time) see a Hardware Summary at the Bottom (press "Pause/Break" Key to Pause the text display).
If that is also not present, re-check if the Bios is set to "PnP OS Installed = Enabled".
(or try Disabled to force detection at boot)
Last resort, move the Card to another PCI slot, and if even that fails, test it in another Machine.
Finally, if all that failed, the card might be defective after all
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Q : Why did 3dfx do the right thing, and made an Add-On card first ?
A : Because people were already gaming in 3D (Software Rendering) with their quite expensive and specialized 2D Cards w/ Vesa 2.0 & often full Windows GUI Accelleration.
At that time, there was no 2D Core existing, that 3dfx could have used to challenge the tremendous 2D Speeds of a Tseng Labs ET4000 or even the ET6000 chip, for example.
People loved their existing 2D Cards just like people today love their 3D Beasts, and sure did not want to exchange it for something far less capable in 2D, just to get the 3dfx 3D advantage.
(Bare in mind there were still alot of non-3d Accellerated Games out there, not even to mention the great, older ones still being played).
I ran my first Voodoo1 besides a Hercules Dynamite 128/Video PCI, which (when I bought it) was quite costly, and was on the performance King of the Hill for a long time
The attachment of a completely separate 3D Card was (and still is) a very easy thing. It just needs to work in the first place, everything after that is almost failsafe.