I was asking myself the same question before I bought my computer.
I went with the two 560 Ti's and my friend with a single 580.
Haven't been any game that he runs faster than my setup.(Few FPS faster in GTA IV but irrelevant.)
My setup is exactly the same as his and it's notably faster in everything GPU related. Not talking about 5-10FPS more like the different between playable and not(METRO 2033).
But yeah, the problem with Vram. Come across it myself in battlefield 3. Could be "fixed" in the final release but what can you do other than lowering AA.
Other than that I haven't come by any Vram issues in any other game.(Except modded GTA IV.)
So i'd say wait for AMD 79xx. Edit: Forgot to mention that everything I wrote earlier are at stock clocks. Overclocked they munch everything spit it out and eat it again then have small cards inside it eating it up also.Edited by EmL - 10/11/11 at 12:17am