I moved to a 2gb 6950 a couple of weeks ago, just to get it out the way.
I wasn't having any problems with the 1gb 4870 and it did all that I asked, when the 2gb 6950 dropped to £205 though, I was forced to act.
Wanted to get in before new revisions of the cards, I assume they will stop the ability to shader unlock at some point. Can't really see that price being bettered for a while either.