See, I was wondering about that very thing - how important is the vram? There is a 2 gb 570 out there, but it's like $400, and I don't know if that makes sense or not...
Well, if you look at it, 1.5gb of vram is optimum for a card like the GTX 580. The problem is, if you were to SLI a pair of them, they would need 3gb of vram on each of them to maintain a relationship whereby the GPU could make use of the frame buffer in an ideal way.
As a single card, the GTX 570 comes with plenty of vram and you won't be bottlenecked in current games. Even in SLI, so long as you game at 1920x1200 or lower you will be fine for now.
It probably won't be until the new year until we have cheap 2gb cards.
Right now is not a good time to purchase a GPU IMO. My advice to you is to try to find something on sale that suits your needs. I've been seeing the 1gb GTX 460s go for around $100 lately, and a pair of them in SLI would crush the 570 for less money. Even a single GTX 460 can handle just about any game out there at moderately high settings.