This topic has been beaten to death; you should search next time.
Graphics card availability really depends on which Mac Pro you have.
Older Mac Pros (2006-07) use EFI-32 ROMs and are thus only compatible with a handful of upgrade cards. I believe the Radeon HD 4870 (Mac version, of course) is the fastest card you can use with those machines.
Newer Mac Pros (2008-09) have EFI-64 ROMs and are a little bit more flexible in graphics cards options. The fastest compatible card for these machines would be EVGA's GeForce GTX 285 Mac Edition. There could be perhaps more options down the line, but that's impossible to predict.
But the biggest thing to keep in mind is that Macs don't (and probably never will) have the vast selection of graphics options available to Windows users. Some of us have successfully flashed (or otherwise tricked through driver injection hacks) PC cards to work in Mac OS, though your results may vary.