I am trying to accelerate anti-aliasing triangles for a Voodoo. According to 3dfx documentation it seems easy, just set bits (19:16) of the alphaMode register to all 1''s and the card should be enabled. This is not working for me though. Is there anyone out there with experience doing this.

Well in gl just us the accumulation buffer, I don''t thinks theres any need for extensions or anything.
But it doesn''t sound like you using gl.