The number of commercial games that run natively on linux may still suck, but generally Linux is an OK gaming OS, if you have good driver for your graphics.
Proprietary nVidia drivers are quite good, and I'm glad if AMD drivers will be good too. In last few years Radeon proprietary radeon drivers have had trouble installing, and had some problems even if you do. So my advice has been buy nVidia if you want to game on Linux and install the proprietary drivers. If you want all open source, get a radeon but don't expect it to run games smoothly.
Let's see what I have for Linux on steam: Half Life 2, Portal, Star Conflict, Dead- Island. They all run well on my machine AMD FX.6300, nVidia GT 430. See if I must upgrade my graphics card for Bioshock Infinite, that should get Linux binaries soon (which I already bought on christmast sale for 7€).