I am not sure which brand of Radeon 4850 I have, but what I did to get it working was went to /System/Library/Extensions/ATI4800Controller.kext. I right clicked and went to show package contents. I then went to the contents folder and then opened info.plist. I added the device ID from my card. After saving the kext and rebooting I can select my resolution. I am using Mountain Lion that I downloaded from the app store. I have an Asus P6T Deluxe.