Having just moved from a 4890 to a 6950 2gig card I can say you will see a performance difference. I'll also add that games look a little better with the 6950. Also effects like fog, smoke, fire, water and stuff looks better with the 6950.
Played Dead Space on both the 4890 and the 6950 and the 6950 had better looking effects. Same with FEAR 2, the 6950 really came through here.
Overall a really good card (the 6950) so I'd imagine the 6970 would be the same except it's faster than the 6950.
You can also unlock the 6950 to a 6970 with a upgrade - Sapphire Radeon HD 6950 2GB Unlocked to HD 6970 :: TweakTown USA Edition
I haven't done mines yet, but I may soon.
Here's my card - Which ATI 6950?