Just curious, but do you really see that much performance difference between the 6870 and 6950? Most reviews/tests I've seen show those cards to be within 10% of each other unless you start to run mega Eyenfinity resolutions when the 2GB starts to be useful.
It would depend on what resolution the cards are being used at, and what detail levels are turned on in games. At higher resolutions (probably at 1920x1200 and up) you should definitely see a difference At 1680x1050 and lower, even if there is a difference in frame rate, both cards should be able to provide more than playable frame rates, so it shouldn't matter.
It's not just the amount of memory by the way; the GPU on the 69xx series is more powerful. The memory comes into play at higher resolutions or when games have very large textures available, but the GPU affects more than that. Check out this page of TR's review of the 69xx cards and compare it to the 6870 scores. The 6870 doesn't come close to the 6950 in Metro 2033.http://techreport.com/articles.x/20126/12
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