I'm thinking about getting a second Palit 580 GTX but I'm not entirely sure if my PSU will run it and the rest of my computer. I have a 3GHz i7, not the newest one, a Corsair TX950W, one SSD, 2x 1TB HDD, 1x 2TB HDD and the motherboard is a Gigabyte GA-X58A-UD3R. I think my PSU should be able to hand an additional 580 GTX but before I shell out money for it, I'd like to double check.
If you calculate on needing at least 488w for the cards alone and then factor in whatever the rest of the system needs you should be able to work it out from there.
You will be fine with that PSU but I'm not sure if it would give you enough headroom for OCing.