I 2nd the i5 2500k, it has almost same gaming performance as i7 2600k. Take that money you save on the CPU and get a better GPU. A better GPU will have the most impact on gaming performance of your PC. GTX 580 or 590.
I just put a system together with an i5 2500k @ 4.5 Ghz / GTX 590 Cassified. I got some great sale prices and only spent $1400, & the system blows any game away I throw at it @ 1920x1080.
-if you can wait the new Video cards will becoming out shortly. AMD 7950, 7970. With Nvidia 600 series in a few months. They will be more powerful if you can wait.