There is an ongoing debate about 2gbs vs 3gbs. Some swear there is a difference in certain games even at 1080p, most notably BF4 gets mentioned a lot. This does not bear out in benchmarks though, so 2gbs would be the recommendation. I do believe my experience with BF4 appeared to feel better with 280x 3gbs vram against gtx680 2gbs vram (comparable gpu's). However, there is most likely other reasons like drivers etc. Staying with facts though, 2gbs is all you need.
Having said all that, i have 280x's now, cause i game at 1440p only. Most will say you can do that at 2gbs vram also. However, hands on the gaming experience is totally different with 3gbs.