I certainly would not agree with the comment that it is under-specced for gaming,the 580 GTX will handle anything you throw at it with all settings on high.
I would include a SSD for your boot drive with a big Sata for your files etc. Change the motherboard to a Z68 as Z68 boards are more likely to be able to accept the new ivy bridge processors when they released, should you want to upgrade in the future.
You will need a decent case of course and an optical drive and windows 7.
Still disagree the i5 2500K can be over-clocked very easily and there is little difference in gaming performance with the i7. The i7 only beats the i5 if you were into video editing in a big way on in applications that can use hyper-threading.
The GTX580 is a very powerful card, I have the Gainward GTX580 Phantom 3072MB which actually beats my twin GTX295 Qual Sli hands down because I run all my games in 2560x1600 resolution.
The i5-2500K Sandy Bridge 3.3GHz is certainly OK for gaming on either a P67 or Z68 motherboard but I don't think you'll see any significant performance gain from an Ivy bridge CPU should you want to upgrade in the future, it is more energy efficient however.
You didn't mention the RAM speed but I'd advise checking the motherboard site for RAM compatiblity because there were some reports of certain G.Skill Ripjaws RAM having problem with some Asus board.
For roughly the same price I'd recommend this 850W Corsair PSUclick here
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