As far as I know the difference is with fps on higher ressolutions. If you use normal 1080p resolution there's not much difference between using a 1gb card and a 2 gb card except of money loss. But in some tests I read, the factory clocked 1gb version scored close to the 2 gb on regular ressolution
also there's no way to flash the 1gb card to a 6970 if I'm not misstaken. the referance model of the 2gb one can be flashed to a 6970
Guess someone else got some better tips than me tho. But I would say that you could save some money by getting a factory clocked 1gb card instead of a 2gb card. That is atleast if you don't consider OC'ing yourself, but I don't know anything about OCing so someone else could prolly take that