I thought you needed two PCI Express x16 slots and that the Mac Pro only had 1 x16 and three x4s. Either way, Apple doesn't really seem to be trying to be gamer-friendly anymore.
The 8800GT is lightyears ahead of the overpriced, underpowered, two-and-a-half generation-old X1900XT, but they're charging $100 more than the retail on the PC-side... and you can't even SLI. I do see the 8800GT as the perfect top-of-the-line card for Apple (as to get a faster card you'd have to move up to the GTX, which is a lot more, and not always faster, if not the Ultra which is obscenely expensive), but the lack of in-between cards is sad (a 2600XT is not a mid-way between a 8800GT and a- oh right.. there's no low-end...)