Guys,
I did a close comparison in a number of current games with a 580SLI bench review from X-Bit Labs. From what I can tell, the new 680 is essentially equal in performance to the 580SLI, and you don't have to deal with all the heat, which in my mind is the major drawback of SLI systems. And money of course (as it cost close to twice as much to buy 2 duh LOL).
Also, I just got my gf a MSI Twin Frozr 680. It's super freakin' sweet. It's got the dual fan design which is not standard on a normal 680, and the parts are military-grade. Bite the bullet and get one. From the reviews I have read, it is the fastest 680 currently made, and would probably do even better in benches than the ones I have read on the normal versions, like the EVGAs for example.
Cheers!!