I'd go the 2GB if you're on 64-bit os, I mean, can you actually give us the price differences? As I can get the 6950 2gb for ~$290 aus, which isn't expensive. And the 2gb is faster and possibly can be unlocked, though apparently from word of mouth sources I have read on these forums from posters that the 6950 does not always unlock and you don't want to bet that it will.
You should go for the one that you'll be happiest with the stock performance of, and if that's the 6950 2GB, then consider unlocking as a bonus.
hope that helps too.Edited by Eskanasi - 3/18/11 at 4:06pm