Only get the 2600K is you're actually going to make use of those extra threads....like video encoding/rendering/editing
while you're gaming or something. If the main purpose is just to game, then the 2500K is plenty and will save you $100.
If you're buying a system very soon, stick with Intel's Sandybridge offerings. There is not much of a reason for you to go with a Phenom II X4 or X6 CPU now at all, especially with your budget. The 2500K is one of stronger chips in the market and it's relatively cheap for the performance it gives. AMD will be releasing Bulldozer some time in August-September, which is to compete with Sandbybridge if you want to wait.
If you plan to overclock, make sure to get another CPU cooler....the stock one is terrible. For cheap, the CM Hyper 212+ is a great choice:
http://www.amazon.com/Cooler-Master-Hyper-Sleeve-RR-B10-212P-G1/dp/B002G1YPH0/ref=sr_1_1?ie=UTF8&s=electronics&qid=1307781992&sr=8-1
The Scythe Mugen 2 is awesome cooler though:
http://www.amazon.com/Scythe-Mugen-rev-B-SCMG-2100-nickel-plating/dp/B002SCPH9W/ref=sr_1_1?ie=UTF8&s=electronics&qid=1307782071&sr=1-1
side question: how much wattage will I need just based on those parts? I think I have a 750W or 850W picked out.
A quality 500W PSU would run that setup. A quality 650W unit would run two GTX 560 Ti's in SLI easily. If you're getting the GTX 570, then you can just go for a 650W unit for more breathing room. Seasonic (X650 Gold), Corsair (TX, HX, AX models), XFX, and Antec (non-Basiq models) are a few quality vendors.