Currently, only TWO GAMES (Crysis 2 and Metro 2033) uses over 2GB of vram with a triple monitor set-up.
There is no need to go with over 2GB if you're only doing dual monitor gaming.http://www.xbitlabs.com/articles/graphics/display/evga-geforce-gtx-670-4gb.html
And with the GTX660, it's more of a memory bandwidth issue rather than the lack of memory that usually restrains performance.
(Only difference between GTX 660 ti and GTX 670 is that GTX 660 ti has 192 bit memory bus and GTX 670 as a 256 bit memory bus and look at the performance hit the GTX 660 ti takes when running the same games...)
Edit: oh wait. you're looking at the GTX 660.
Either way, that GPU isn't going to cut it gaming at higher resolutions on max settings so you don't have to worry about ever needing over 2GB of ram on it.