I have got an XFX 5870 1 GB card that I put on a Mac Pro 2,1
Pros:
1) Cheap. Bought on NewEgg and was way cheaper than an Apple version.
2) Fast. Can run 3D and video applications faster than ANY other official mac cards
3) Two dual link dvi connectors + HDMI. I have two old 30 inch monitors that have dual link dvi connections. If you buy an apple version, you get two minidisplay ports and one dual link divi port. I am so glad I didn't have to buy an apple adapter from minidisplayport to dual link dvi. Read the reviews from the apple store, it is a craps shoot to get a working device. May not be an issue for you if you don't have old, large displays.
Cons:
1) Non-Apple. I had to put it in the mac pro, run windows, flash it with a rom from someone else that mixed EFI code in the BIOS of the card in order to use with Snow Leopard. This included also getting a special cable to power the card. (The card comes with an extra power cable that won't work in a mac, had to buy it in ebay)
2) Can't see boot screen. I don't know if this is with any flashed card, but I can confirm this is still the case on a new MacPro 5,1. I boot and can't see the apple logo, which is a non-issue. But if you press the Ctrl key while booting, you can choose a different boot volume, but you can't see it. Is like flying blind. Still, not a deal breaker for me, but may be for you.
3) Game performance on Snow Leopard is subpar vs windows. Sad but true. Pick any game in both platforms it will perform better in windows. For example, I can run StarCraft II on a 30 inch screen full resolution (2500x1600) in win 7 and get 30+ fps. In Snow Leopard? I can't get resolution that high and still get 30+ fps, more like 20~25 fps. Same with Civilization, Half life, etc. (OK, not really a con between flashed vs non-flashed cards but thought you should be aware of this)
Overall, I am glad I bought it! Definitely worth it!
