The more vram you get, the higher anti-analysing you can set and higher the resolution you can play on, 1080p (1920x1080) is the most common res now and 2Gb is fair enough for this (in this generation of games at least). However, some games, although the vram is not completely used, other factors in your gpu will be limiting that the gpu cant run the game smoothly, even if there is free vram. What is meant, that if you are playing at 1080p and want to turn on some AA and have fun with smoother edges in the game, get the 2 Gb, it will also be more futureproof for the next-gen games, although i suspect this gpu will survive in the next-gen games @ ultra settings. Games that demand huge vram will require the 2Gb, far cry 3 for example
http://www.guru3d.com/articles_pages/far_cry_3_graphics_performance_review_benchmark,8.html , this shows that far cry 3, will use a 1.2 gb of vram in ultra settings on 1080p. While crysis 3, which is more demanding, will use 2.2 Gb
http://www.guru3d.com/articles_pages/crysis_3_graphics_performance_review_benchmark,8.html , note 2 things, that the AA is maxed out in values i said, and that you can check your gpu's fps in these reviews as well. Finally, i would go for the 2gb to be more futureproof and get more AA out of the games.