I'm going to offer a completely contrary opinion, based on the fact that it will be a kid's computer. My son, 7 years old, has a selection of games that drove me nuts when I tried to use a Nvidia-based TNT2. This game required this driver, that game another, never got some of the DOS games to run at all. Patches, updates, crashes....Maybe the newer cards are better, dunno....
Then I installed a 3DFX Voodoo 3, and all the headaches went away. It will run any game we ever threw at it, no tweaking, no patching, no fussing around. Not the best framerate or resolution, but he doesn't care, it just works.
I dread the day he comes up with some game it won't run, I'm gonna miss 3DFX.