The i5 2500K performs on par with the i7 in gaming currently because no game was optimised to utilise more than 4 CPU threads. When upcoming games will be able to utilise even more threads, the i7 2600K will outperform the i5 easily because of hyperthreading.
An i7 2600k would be overkill for gaming, I would stick with an i5 2500k and save yourself a hundred bucks.
Go with the i5 2500k for a processor.
Choose a z68 motherboard such as:http://www.newegg.com/Product/Product.aspx?Item=N82E168...
I would go with a radeon 6950 graphics cards ( go with a sapphire branded one)
For a case go with something you like. I prefer cases with good cable routing.
Powersupply: Go with a 650w ( Corsair, Seasonic, or non earthwatts antec supplies)
Hard drive wise go with a 500gb sata 3 drive (western digital or a samsung spinpoint) and buy an inexpensive solid state drive to use as a suppliment to the mechanical hard drive.
Ram: get a cheap 8gb (2 x 4gb) set from gskill or corsair