Extra memory only increases performance when you are playing at resolutions much higher than 1980x1020 (multi monitor setups mostly). It has to do with the fact that your screen has to load alot more crap onto it faster. At 1980x1020, there is absolutely no difference between 1gb and 13232gb of ram on a card as anything above that 1gb will go unused.
IMO go with the GTX Twin Force III. It is cooler, factory oc'ed and cheaper.