Baggins: Stixmaster's suggestion doesn't actually access the 3dfx mode.
Direct3d can't access 3dfx mode.
Also you actually have to literally patch to the 3dfx mode into the game. It's not included in the GOG download. The patch and special exe are on the 3drealms website.
Stixmaster doesn't explain that you have to patch the 3dfx exe into the game.
Stixmaster's suggestions will get you higher resolution modes and widescreen though.
3dfx though is stuck at 800x600 limitation as far as I remember. But you get some other graphical bells and whistles including texture smoothing (smooths over pixellation on textures), special transparencies, improved water and reflections, explosion effects and color lighting.
You'll notice I posted the issues I came across in the other thread, when I was trying to use a 3dfx enabled version of Dosbox..
Ah, I see. Me personally, I just use either the standard non-3DFX version in DOSBox, or I use the ProASM Shadow Warrior Windows port. It has been a while though.