If your desktop will be purely for gaming, you should look for an Intel CPU.
See the benchmarks here: AnandTech Portal | The Vishera Review: AMD FX-8350, FX-8320, FX-6300 and FX-4300 Tested
Also I´m not really a fan of already built systems, because there´s always some trade-off with RAM, grafic cards and the like. If you´re capable, build your own system from scratch. Saves money, too.