For the record, I have run an ATI Xclaim (Rage II+/Mach64 graphics), a Formac Proformance III, and a 3Dfx Voodoo 3 2000 (not all at the same time, of course!) in my 6500. The 6500 was equipped with 128 MB of RAM. It runs Myth II with the Voodoo 3, but only just barely. It still labors quite a bit. Once I dropped a G3 upgrade in it (replacing the 256 KB L2 cache), Myth II was a bit more peppy, but it still lagged in high complexity scenes. I tried to play Oni on this configuration, but that was pretty much a hopeless cause. A real G3 (even a Rev A 266 MHz G3) easily smokes the 6500 G3/400.
You might be able to play Quake (1) or Descent pretty well with that configuration, but I wouldn't count on playing anything more complicated than that with any satisfaction.
On the other hand, using any of those cards as a (not-so) basic framebuffer is really nice. They all allow the use of really big displays - I drove an AppleVision 1710 at 1152x876x24 with all of them, and they all accelerate 2D performance so things like scrolling Finder windows and text documents as well as some Photoshop work are accelerated by the 2D hardware on these cards. In my experience, ATI cards have the best 2D performance if you are looking for a combo 2D/3D card. For pure 2D, the TwinTurbo 128M8 (the 8 MB version) is pretty good, but the Number 9 is even better (though VERY hard to find). Both claim to do 3D, but they don't really mean it. ;-)
The sad reality that everyone knows but few speak is that PCI Macs just suck at gaming. The 50 MHz (at best) system bus and PCI graphics just aren't up to the task of gaming in the modern sense. Sure, all these machines will run sprite-based 2D (or "2.5D") games that are largely CPU-bound like Marathon, but they won't play real 3D games well. Even with G3 upgrades, these machines are still crippled by the slow bus. It wasn't until the G3s came out with 66 MHz and 100 MHz busses that things started to get interesting, and gaming didn't become serious until the AGP G4 Macs came out. It really doesn't matter what PCI Mac you've got: whether a 7500, 4400, 6500, or 9600 MP, gaming just sucks.
Still, if you absolutely have to give it a go, do yourself a favor. Get at least a Rage 128 or Voodoo 3, a 200 MHz 604e or better (preferably a G3 - G4s don't really have any advantage here), and at least 256 KB L2 cache (more is always better, but none is a waste of time and electricity). You can ignore this advice and do your own thing with lesser specs, but don't get upset when you aren't satisfied with the performance...
Peace,
Drew