Sa hello to the list of graphics cards u can use in your Mac Pro !
NVIDIA GeForce 7300 GT
ATI Radeon X1900 XT
Nvidia Quadro FX 4500
ATI Radeon HD 2600 XT
NVIDIA GeForce 8800 GT
NVIDIA Quadro FX 5600
Kash is quite correct about apple only providing drivers for a very few graphics cards, but i believe there is also a slight issue with having a card which is compatable with the MAC systems EFI BIOS... im not 100% sure on this point, as iv never tried sticking any card except the Mac approved ones into my machine. But iv been led to believe that these cards are different in some way to that of the standard off the shelf models.
For example the 8800GT which i have, is a Mac dedicated product, u cant stick it into a standard windows machine... and following this logic is why i dont think just any old PCI-E graphics card will work in the Mac Pro (even in Boot Camp Windows)...
[But... as said, im not 100%... wiser apple guru's than me might have a better idea !]