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I (this is: Mark Danks) have a Voodoo2 card, which runs fine under WinNT. 

I use the OpenGL beta driver from 3Dfx at work all the time without any problems and, except that the Voodoo takes over the full screen, it seems to work fine. 

You will need to download the OpenGL Beta driver from 3Dfx's web site at http://www.3dfx.com and put the !OpenGL32.dll into the same directory as pd.exe (NOT gem.dll). 
Debugging patches is much easier if you have two monitors, one for the 3-D card and one for the 2-D card.

IMPORTANT: You **must** set the environment variable 'GEM_SINGLE_CONTEXT = 1'

to make the Voodoo card work. It will make a window 640x480 (which is the correct size for TV video out on my Canopus V2 card). On WinNT, right click "My Computer" and go to "Properties". On the "Environment" tab, you need to add the variable 'GEM_SINGLE_CONTEXT' with a value of 1.
Resizing the GEM window with a Voodoo card is not a great idea. The Voodoo card can only display certain window sizes and will clip the graphics.

For the tech heads in the audience...

 I create an OpenGL context at startup and never actually display its associated window. 

 This means that GEM objects can create display lists, call OpenGL commands, etc. in their constructors, even if no window is actually being displayed. 
However, with the Voodoo card, there can only be one OpenGL context. 
So, instead of creating one context and just holding onto it in the background, I create the normal GEM window and associate the OpenGL context with it...
and the user can never destroy or close that window.