The Radeon 5770 and 5870 are better and show boot screens on All Mac Pros. You can buy a PC version and flash them to mac (save lots of money), buy one pre-flashed on E-bay (save less money), or buy one from Apple.com (waste tons of money!).
However, if you don't need boot screens (ie you have a video card you can use at the same time), I'd highly recommend one of the working NVidia cards.