It's more dependant on the res and it really only shows on triple screen setups (5760x1200 etc).
Hence 3gb models.
At 1.5 the limitation is the ram.
(Tests were at 2560x1600, but say tripple 22" at 5060x1050 is normally 5million pixels, where as a single 30" is 4.1 million so there's only 25% difference in pixels, which is a reaosnably accurate way to determine the difference in vRAM usage between them depending on FOV and other factors oh i didn't say above, but also tested BF3 in both setups but not objectively with FRAPS just online to see if there was any visible difference, none could be seen, hard to judge online though, frames vary wildly from 40fps to 120fps)