OCing 4850

Will the Catalyst Control Center allow me to do a good OC? or is there a better way?

I dont know for sure but have you checked on riva tuner to see if it will handle both your cards?

Folks play around with the BIOS. That way you don't have to set anything any more and it may be stabler. However, you can't easily undo the change. A good thing about BIOS tweak is possibly not having to use CCC, which sometimes makes for runtime errors with .NET or C++.