I'm sure if Valve picks AMD then they'll go for a series that's well supported. It seems to me the HD5000 series encounters the least problems and performs the best of all AMD GPUs on the radeon, radeonSI, and catalyst drivers. I personally own a HD5750 and I never encounter the problems people gripe about with the catalyst drivers. The only thing that doesn't seem to work great is vsync, but X in general has bad vsync problems.
Originally Posted by Dukenukemx