A single 7950 or 7970 is the way to go right now (translation of the above). The 580 belongs to the previous GPU generation and it's not worth the cash. Alternately, you could wait a few weeks and see what Nvidia's Kepler (600 series) has to put up against the 7900s.
IMO it's almost never a good idea to go Xfire/SLI in a fresh build. It's a great upgrade, too cost-effective and easy to throw away from the start. It's true that Xfired 7770s will get you better performance, but then you'll have to buy a whole new GPU setup with your next upgrade.
Your chipset and CPU have nothing do with each other. They're all compatible.
PNR, it doesn't matter how much money you have to spend. The OP's budget is what matters.
It depends on the setup and some people are more sensitive than others. Anyway.
Personally, I think it'd be worth waiting a few weeks for at least the 7800s, if not Kepler. A single card would be good, but the latest generation's high-end single cards, as PNR says, are currently kinda out of the OP's range.