Make sure you remove any trace of the 3dfx drivers, (that means going into the registry) if even one file is left over and the registry trys to or does load it, it will slow down the new card.....3dfx drivers are notorious for that, bad thing is, there was never an uninstall for them, so it must be done manually.
so make sure you do a search of your computer and the registry, find all the files that begin with "3dfx"...these 2 things I gaurantee will remove all 3dfx drivers from your system.