Ill be honest, just look up toms benchmark review of the i5 2600k and you'll see how much of an improvement it is over some of the high end amd parts. I was a fan of amd for years but couldn't deny how much better the intel series was with sandy bridge. I almost had a heart attack when i saw how poorly bulldozer performed.
Well, while most poeple say yes... I would say no... Worry about that later if you choose to upgrade to even better things. 2 580's in SLI pull about 525w from the wall on full load... You'd be pushing it... but no, I wouldn't say you have to upgrade your PSU.
Nevermind about what I said. Lol, P07H34D was correct. I agree with him. That 600w would serve decently for you right now, but if you choose to add a second card, you're goana need to upgrade. An 850w 80 plus certified is the sweet spot for 2 graphics cards and an overclocked system.
Btw, Don't forget to choose a "best answer" when your problem is solved (P07H34D seems to have put a lok of input *hinthint*). People appreciate it
Um.. Search it up on YouTube. Just search up "Newegg how to build a pc". And its a great building guide. You might as well watch the whole thing as it's all useful. Otherwise you can just search up "how to build a pc" and you'll get a bunch of guides.