I have a 4890 1GB that I was planning on replacing soon. I was looking into a 6870 1GB. The only reason that I was turned off to it was that I love GTA IV and the 6870 only has 1GB of VRAM. GTA IV uses a huge amount of memory of course. With my next upgrade, I want to be able to max it out. If the 6950 actually comes out before christmas and has 2GB of VRAM (which is rumored), then it will be the clear winner.
I was curious, however about the GTX470 with 1.25GB of VRAM. I know this wouldn't allow me to max out the game, but would the additional 256MB actually help a significant amount? Obviously, it is a very powerful card at a decent price. And GTA IV is NOT the only game play or care about. It just concerns me when they list the 1.25GB as restricting in some games, like Black Ops.
Other than resource hungry games like GTA, how big of a frame buffer do you really need for today's and tomorrow's games?
The problem is that GTA IV isn't optimized well, even top notch systems have problems with it(low FPS issues mostly);however,going from 4890 to either 6870 or GTX 470 will give you a good boost in performance and as dipankar mentioned,1GB VRAM is enough.
Make sure you have a good CPU too,GTA IV benefits from quad core CPUs.