FWIW the later ATI Rage cards do not particularly like working in onboard video Macs such as the 5x00 and 6x00, and worse with G3 upgrades.
It's my understanding that this is a 5500/6500/TAM problem, and
not a 6400 problem, because the onboard video on the 5500/6500/TAM was, itself, an ATi Rage. (Or perhaps just because of weirdnesses with the platform, I've heard both explanations.)
Those cards all work fine in the 7000/8000 series Macs, and I believe also the 4400, which use the same graphics the 6360/6400/5400 do, so I have no real reason to believe it would be a problem in a 6400. "Stick a Rage in it" was pretty much MacAddict's recommendation for the 6400 when they were new, too.
Heck, I don't think this is a good idea, but as far as I know nothing is stopping a Mac PCI Radeon 9200 or 9250 from running in a 6400.