I have an XFX Core Edition 6770 that I'm trying to get to work with my Mac Pro and Mountain Lion (10.8.4). How do I get it to work? I see that it is connected in the Expansion Slot Utility (1st slot): But I plug my monitor in (via a DVI-VGA adaptor) and even after boot, the screen doesn't come on. Suggestions? I'm kinda stuck here.