Just buy/build a PC. A Mac Pro is a total rip off if you're using it just for games. I'm building up a crazy vista gaming rig, and buying a mac mini for everything else. It's the only way to go, IMHO, unless you run hardware intensive, mac-only software.
Now dont get me wrong: I LOVE, LOVE Mac OSX, to the point where using windows makes me cringe. However, I'll be the first to admit that Macs SUCK for gaming. Unfortunately, it will always be that way. Apple makes a ton off their hardware, and letting people upgrade, or *gasp* building their own system would cut into their profits. There is a very marked benefit to having a lock on the hardware, though: no driver conflicts, etc.
In the end, I really don't mind. Sure, I have to shell out an extra 900-1000 bucks for a mini and monitor