Yeah, for a 6600GT there is no difference. Only with higher end cards does it matter, cause then you can turn on all the effects that use video memory. There is a few rare cases in older games where you wouldnt be able to turn everything to max, even though the card would be capable of handling it, simply because it doesn't have enough video memory (Max Payne 2 is the example I am thinking of). But for basically any game out today, 256 mb is plenty. Future games will utilize more, but again, you will need a high end card to be able to enable enough effects to use that much memory.
That's why a 6800 512 Mb is a totally pointless card.