MacPros need EFI firmware on their graphics cards, PC graphic cards have been found to work in BootCamp but only display stuff once the Windows drivers have loaded.

Brief article hear:
http://www.hardmac.com/news/2006-09-10/

Maybe once the PC world moves to EFI we will no longer have anything but driver differences between PC and Mac Graphics cards.
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Burt
I break stuff.