For gaming a 2500K would be a better choice. For whatever reason the x6 even lags behind x4s in gaming.
I think starting with one GTX570 might be good idea. You're then not paying a premium for the GTX580 and a step above the GTX560 without too much wallet pain. You could always add a 2nd card right away if the performance wasn't up to what you were expecting.
Or just go GTX580 SLI + 2500K upfront if you got the cash.