The SLI would probably help games when playing them at higher resolutions, though to be honest, most people around here don't think SLI is worth it.
I would get 4 GB of memory... but with current games it might not show much improvement. Many games simply don't require over 2 GB of memory. Though, I have seen one or two recommend 3 GB... Your call on that one... If it's too expensive to get 4 GB, it likely isn't worth it.
Haven't used Vista yet... so can't answer your last question.
Unless you specifically need a gaming machine that you can carry around... you might want to consider a desktop. You'll get more performance for a lower price that way...