in bf3 i tend to use 1.5GB of my 2GB card at 1920x1080, if your going to play newer high end games on triple monitors, go for the higher capacity card.
if your only ever going to be playing games on a single monitor and using the other 2 as ancillary monitors then the 2GB will be fine, but if you wanted the option to triple monitor in a game the 4GB would be more suitable.