I want to use my 3DFX Voodoo2 instead of my ATI Rage PRO 3D AGP card becouse the 3DFX is faster (tested in Quake 3) but I don´t know how I should do to use it ! I have made a textured cube who is mirrored in a flat red floor using a pixelbuffer, but it is verry slow when I start it, I don´t know why becouse it is only 12 polys in the cube and 2 in the floor and the computer should be able to render this at a good speed with software. there is no lighnig at all and I use DisplayList to draw the cube and flor one for the cube and one for the floor.
see if your color mode is 16 bpp. As u may know, 3dfx accelerated pixel formats are only supported in 16bpp - see DescribePixelFormat() to see if the choosen pixel format is accelerated. Otherwise your ogl calls will be handled by the Win32 generic implementation - maybe this is the reason for the slow performance… 
ATI accelertion, on the other hand, only happens in 16 and 32 bit mode. 8 and 24 are software only. (doh!) If you run a Quake game with 8-bit texturing on, it’ll run faster on anything but a ATI card. 
I don’t know if this applies to your Quake III test (does it allow 8-bit colour?), but I know it does to Quake I and II.
I run my openGL application in 16bit color mode and in Q3A I also use 16bit color mode but my cube is still slow… I have watch some demos from demopartys, they use opengl and they are fast, even when I use my ATI card!
You also need to link to 3dfx’s own standalone opengl dll.
Read this: http://www.opengl.org/developers/code/features/drivertypes.html
and this: http://members.home.com/borealis/opengl_usingq3.html