You might have a problem finding drivers for XP, since nVidia bought out 3dfx and basically killed the line off. Some of the fomer developers have been keeping the drivers up to date in their free time, or were last I knew a few years ago.
Assuming you can find drivers, there shouldn't be much of a problem with the video cards conflicting if it doesn't work like you want.
Just remember that all 3dfx cards (that I know of) only accelerate 3D graphics at the 16-bit color resolution. So if you run 24 or 32-bit on your Radeon card, it might default to that even if you use a different hardware profile.
I have a Windows XP machine with lots of memory and more hd space than I can fathom.
I have a Radeon 9600 Pro that is unable to run an older 3dfx based game, even with the latest Glide wrapper (crashes out).
Can I safely install an old 3dfx card into a free PCI slot? I understand that XP will allow you to set up different hardware profiles. Would this allow me to select which card to load on startup?
Thanks in advance for any help.