The 256mb on the sucky cards like the 9600's and say 5700 or something is just a gimmick to get people to buy them, the average person who sees say a 6800GT with only 128mb of ram and a 9600XT with 256mb of ram will prob buy the 9600XT.
The 6600GT with 128mb should be more then enough, remember 256mb only helps if you re able to crank the resolution graphic details and AA/AF up. The 6800GT's in say Doom3 started use all the 256mb only when the reso was turned up to 1600x1200 with ultra high details and like 4xAA 16xAF.
The 6600GT wont be playing Doom3 @ those settings because itll lagg out way before. Basicly 128mb on a card the speed of the 6600GT and lower for each ATI and NV is perfect, only the really high end cards like the 6800GT or X800PRO or high will really benifite from the 256mb. at least from what i understand, someone correct me if im wrong