I built my computer similar to these guidelines below, with slight differences such as 16 GB RAM instead of 8, a cheaper hard drive before the Thailand flooding, no graphics card, and Windows 7 Professional. But this build below outlines a fine gaming computer with mostly second-best materials.
A 6850 benchmarks pretty high among graphics card, don't need to spend more unless you're seriously interested in the most extremely graphically intense games like battlefield with max settings and stuff. Then you're looking at GTX 560+. But for everything else, a 6850 should be fine and i5-2500K is significantly weaker than i7 2600K.
.... 6850 is not good enough for 1920x1080. It's a 1680x1050 High settings card at best, it's basically the 5850 and my 5850 somewhat struggles with BF3 at this res on high settings, also there are more demanding games than BF3 that most people play.
The i5 2500K vs 2600K is not SIGNIFICANTLY weaker, in fact the difference is just 1-2% in gaming and the 2600K only has advantage in multi-thread programs, and even then the 2500K is more than sufficient, worth the $100 saved and put towards the GPU.
Most gamers would need more than a 6850 and don't need a 2600K. 2600K would only really be justified by say Adobe Creative Suite or Maya etc progs.