As some1 said above, with mid end cards, such as the 6600GT, 9800pro, 9600pro. 128 is enough memory, as others said, the 256 is just to make more money. With high end cards, like your 6800gt, x800pro and above, 6800ultra.. 256 is basically a necessity which is why you see near to none that are 128.
so, mid end card, look for 128 nowadays, 256 will become standard soon, i reckon.
higher end card 256 at least. 512 will be out soon,