Mac's are very limited in the Graphics Card department...
#1. You need a Mac version...
Comes with EFI in the video card.
#2. All AMD HD69xx, have that EFI except the 6970 & 6990.
#3. All AMD HD69xx need Lion drivers, except 6990.
#4. HD5770 & HD5870 are compatible with Snow Leopard...
#5. AMD FirePro D300, D500 & D700 needs more than Lion.
https://en.wikipedia.org/wiki/Mac_Pro