http://www.kexts.com/view/95-radeon_4850.html (4850 package)

andddd

http://www.kexts.com/view/6-evoenabler.kext.html Enabler

Unfortunately I am not sure what I did with my 10.6 ATI kext, but there should be one there regardless when you install, just change the device ID like usual after doing the 4850 install and you'll be ok. I only uploaded the ATI controller kext so people could be lazy about editing it manually for 10.6

To enable your ATI graphics with proper resolutions and stuff just edit the ATI controller like you would after each update