I have a fully working snow leopard 10.6.2.
At the begging it was running 10.6.0 with a Radeon HD 4890 with the following kext: ATI4800Controller, a modified ATIRadeonX2000, EVOenabler.
The video card got broken, I installed a Radeon HD 4650 and used ATIRadeonX2000.kext and a chameleon beta booter fron Netkas site and It worked but system begun to be unstable with random hangups (no screen of kernel panic, just stopping fans and messing up video).
I upgraded to 10.6.2 ... not better
Now I'm testing the system with TechTool Pro 5 and ... guess what? Leopard believes that I still have a Radeon HD 4980!!!
So, here's the answer: how do I get rid of it form system and begin using 4650 kexts? You know, everything is ready for a system reinstall but it wouldn't be ... how to say ... hacker