If you're contemplating between the two - the biggest difference is that the i5-2500K can be overclocked, where the i3-2120 can't.
I wouldn't bother with the 550TI or the 460 at this point - both are becoming pretty moot points with the NVIDIA 6XX just announced and the new 7970 and 7950 owning left and right.
As far as PSU goes - that's the most important part of the system and you don't want to go cheap in this area. You want the best and most stable PSU you can get for the money, and the OCZ Modstreamx are a tier 3 PSU at best. A tier 1 would be something like this: http://www.newegg.com/Product/Product.aspx?Item=N82E16817207013