1) you're cpu will NOT bottleneck a 580 SLI set up - all you're really doing is getting a 590 without as much microstutter with that configuration (believe it or not).
2) that corsair PSU is a great choice and though folks are apprehensive of a coolermaster; the silent pro series are better than the usual garbage the put out. however the AX 1200 has slightly better efficiency across the board.Reviews - Corsair AX1200 1200WReviews - Coolermaster Silent Pro Gold 1200W
a single 580 will use ~ 250 watts each so you can drop down to a 1K watt PSU without having to worry. but if you are looking for an overclock to benchmark with; a bit higher won't hurt.
now take this with a grain of salt: the way i understand it, the reference 580/570 use the same pcb and power phases and the MOSFETs on the power phases can get fried if you push an overclock too high (1.1 volt).
i have been able to bench heaven @ 870 core/2000 mem 1.038 volts and play cyrsis2 850 core/ 2000 mem @ 1.038 volts stable.