GTX580 all day. The only reason I decided to go with my GTX560 Ti 2Gb was because I was on a budget. I've got it OC'd up to 977 core clock, 1954 shader clock, and 2205 memory clock. Runs the game very well, only dipping down to about 27-28 fps minimum over land with around 20-25 planes in the sky. A very good buy considering it's almost half the price of the 580. But never-the-less I would very much like to get my hands on one.
Also, I went with a SilverStone 750w PSU. Extremely good quality, 100% modular, and was only $20 more than the standard Corsair 750w at $130 USD. Highly recommended.