I've tried a lot of methods to make Snow Leo installation boot with my HD4850.
There are a lot of solutions involving kexts (ATIRDRV.kext, ATINDRV.kext, evoenabler.kext, gfxstring ) but none of them seems to work with Snow Leo installation.
I think the problem is that Chameleon loads Snow Leo installation kexts instead of the ones in /Extra/Extensions folder.

Did anyone manage to boot Snow Leo installation using an HD4850?

--David