SLI gtx570 will do better then 1 gtx 580.
here is a review that shows it: GeForce GTX 570 SLI review
extra memory is good if you are running high resolution. by high resolution i dont meen HD. i meen by having dual or 3 monitors for gaming.
then the extra memory really is gonna help. in some cases you will get better performance with extra memory with only 1 screen at 2560x1600 resolution, but it is very little.
i will not waste money on it if you are not running somthinf like 3d sorrund vision.
i did find a review that confirms my theory about almost no performance increase with 1 monitor at 2560 x 1600.
EVGA GeForce GTX 580 3GB Review - Page 10
i just saw the post above. he posted some seconds before me.
he explains it well.
i see no problem in upgrading to the 580, it is a little expensive but you also get sic performance. but personal i think that instead of buying 2 gtx 570 at once i will diffidently get the gtx 590.
but if you are gonna buy 1 card first and then one more for sli later, then i think the 570 is a great choice.
as nbrikha sayd. the 580 is a little expensive for what you get. and by adding 2 of the 580 is just even more of an expanse.