Old thread, but relevant again..
I didn't know it until last week but Apple has put compatible drivers in for the Radeon HD 6870 video cards in the Mac Pro. But only in OSX 10.6.8 and Lion 10.7
I just got the below one from Newegg in my Pro 4.1 and it's working like it was native to my system..
XFX HD-687X-CNFC Radeon HD 6870 2GB 256-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card