1. Edit the Info.plist of your ATIRadeonX1000.kext and ATIRadeonX1000GA.plugin and add in your device id's.
Mine for example is 7100 (device id) and 1002 (Vendor ID) (You can get these from System Profiler)
In ATIRadeonX1000.kext it is added under the IOPCIPrimaryMatch section as
In ATIRadeonX1000GA.plugin it is added in two places to Info.plist but now in the reverse order
Mine I added like
2. Now download IONDRVSupport.kext and ATINDRV.kext from Ramm's thread here (Useful files posted to the forum thread)
Edit the Info.plist of ATINDRV.kext and change
ATI,Wormy to display
IONDRVDevice to IOPCIDevice
3. Copy the kexts to your /System/Library/Extensions (As root, do sudo -s to get root)
chmod -R 755 ATINDRV.kext
chown -R root:wheel ATINDRV.kext
chmod -R 755 IONDRVSupport.kext
chown -R root:wheel IONDRVSupport.kext
4. Delete /System/Library/Extensions.mkext. Restart and boot with the -f flag.
You should now be able to change resolution, display depth and rotation in Display Preferences. System profiler should also report your total Video RAM correctly (512 MB here), and also show that you have QE/CI/CA enabled.