Hey, sticking with that rig, 1GB version would be enough, because you aren't likely to be using ultra-settings in game (though nothing stops that).
2GB+ of VRAM is only needed for high texture levels with some AA. There are a number of games whose performance drops heavily if the card runs out of VRAM, but usually turning off AA can fix that. If AA is not on, reducing textures from their highest to next highest would allow you to avoid that bottleneck.
In my opinion, get the 2GB version. It isn't a lot more, and can reduce any headaches at high resolutions (though the rest of your would hold you back.)
Do you know which brand you are getting?