no clue it's all speculation really.
I doubt you'll NEED a 512mb card honestly. Just like any hardware it'll take a while before it becomes normal to the point of 'wow you have to get this to be able to play the game'
Look how long 64mb cards were around, even 128mb is still good for todays games, so 512mb will just be for those "oh man I gotta have the top'
the 6800GT will last atleast a year or more IMO, it will be the last card I buy for quite sometime, because the next card I get will HAVE to be PCI-E in which I'll need a new mobo, chancse are a new PSU and AMD may be using DDR2 at that time so perhaps even DDR2 memory, who knows.