+1 for the 1gb
in higher rez the textures that should fit into the vram get bigger and therefore need more ram, additionally graphical effects like AA etc also take their fair share.
If the ram cant hold all of the stuff, the rest will be pushed to the main memory or, even worse, to the swap file on HDD. This will practically kill your framerates, you'll be lucky to get like 10% of what the card would otherwise be capable of.
the point where 512mb is too little is usually somewhere around/between 1680x1050 or/and 1920x depending on the game (and the AA etc settings)
in the recent reviews you can notice how the GTX295 literally dies in some games when going from 1900x (typically 24'') to 2560x (30'') while the 4870x2 only takes a 'minor' hit. thats the effect of having 'just' 896mb of vram on the gtx versus 1gb on the x2 per core.
(but if the needed amount of vram is say 400mb then there wont be any performance difference between the 512 and 1gb cards. in that case the extra ram wont be used at all)