2gb. You wont need 3gb if your planning to run at up to 1080p. As stanththeman123 said, you can also go 7950, better card than 660ti, especially at higher res. And may be even cheaper or same price point. But you would miss out the physx of nvidia. So it would really come down to brand preference. if your into nvidia, go 660ti 2gb, if you can afford 670, then much better, but since the choice is only between the 2 660 ti's, go 2gb. if not a fan of nvidia, better get a 7950.