590 is a bit more performance/money, less energy and heat. 2 580s in SLI are faster than a 590 though.
IMO, go for the 590.
You're mistaken. The 580 is definitely better price/performance wise.
Also, there's really no reason to get a 2600K, unless you already have it. The 2500K performs roughly equally in gaming for $100 less.