Actually if you look at reviews comparing the 6950 1gb and 2gb models at 2560x1600, the difference isn't all that much. Usually less than 5fps and sometimes basically even.
That said I do think more VRAM the better for huge resolution, but I wouldn't exactly say 1.25gb is going to suffer in a big way.
Anyway I think that no matter what you do, you'll have some buyers remorse. Buy big now and in 5 months there's something much better, or buy cheap now and probably be unhappy with it, maybe buy a second card, and again in a few months regret buying them due to the better new stuff.
If I were you, I think I might just go with a 120hz display and a 560 Ti and when the next gen come out, think about a couple more monitors and a more powerful card. Either that or just get the 580 because it's a beast of a card and should pump enough juice for 3 monitors - although maybe not at absolute max settings in every single game.