1) For gaming, the graphics card is all important. I like the GTX580. It is a bit expensive, but you can't do better if you game at 1920 x 1200 or less. Even 2560 x 1600 is good.
2) For gaming, the 2600K is not necessary. Few games use more than two or three cores. The hyperthreads on the 2600K are not helpful. You can OC either chip to about the same levels. 4.0 is trivial, and there are very few games that need more. Spend the $100 saved more productively.
3) For a single GTX580, you only need a quality 600w psu with 42a on the 12v rails. A 6 and a 8 pin pcie connector.
Even the GTX590 needs only 700w and 50a.
A good 750w unit from PC P&C is only $95 after rebate:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817703027
4) For gaming, a 8gb kit of 2 x 4gb is typical, and more than you need. Expect to pay $80 to $95. Speed and latency do not matter much with sandy bridge, but there is such a little price premium for DDR3 1600, that I would go with that:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231314
5) If you love the case get it. But, you do not have much to put into it, so you could do with something much smaller or cheaper.
6) Get a sound card only if you have high end speakers, and you can tell the difference. I suggest you try the motherboard HD sound first. You can always add a sound card later.
7) Motherboard looks reasonable to me. But, with the introduction of the Z68 chipset, consider going that route. In particular for the SSD cacheing option.
I have not researched individual Z68 boards, but there is a Gigabyte example for $120:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128501
8) Get a SSD into your budget. You will not regret it. Today, I think Intel is the most comfortable way to go. They have had lower return rates.
No need for the high priced 6gb versions. They show well in sequential benchmarks, but that is not what we do. Small random reads and writes at low queue depths is what is important for everyday usage. Look at the Intel 320 80gb or 120gb versions. You will pay about $2 pergb.
If you go the Z68 route, you could use their 40gb version to cache your 1tb drive. Performance will be in between a hard drive and a ssd. Still very good.
You could get
-------good luck------------