are you using both monitors to game? meaning 3D surround/eyefinity, where the game is simultaneously rendered on both monitors together in something like 3960x1080p? Even then, 2GB should be enough, 4GB should only be considered for three monitors or above and even then it's a bit of a rip off.
as for SLI, keep in mind that VRAM do not stack, the two cards process frames separately, thus having two 2GB cards still gives you 2GB of effective VRAM