If you already have 1.5 GB card(s), I don't think it worthwhile to upgrade to 3GB versions for 2560 x 1600. I haven't.
But if you just now upgrading to the 580s, and haven't purchased yet, 3GB versions would seem to make more sense.
It made more sense iny case to go to the 3gb 580's than the normal ones.
They should be here on Thursday, I'm super stoked. I will post some benches at 2560x1600.