Could you expand on this? In other words how does the GPU speed limit the amount of vram usage?
Sure.
Example:
Take a game like BF3.
A 6950 2gb at ultra settings will use 1.3gb of memory yes, but a 6950 2gb will be running at a non playable 27 fps.
A 6950 2gb running some high and some ultra settings will use 990mb of memory at a playable 40 fps.
But if you add the horsepower of another 6950 you can run the game the ultra settings , using 1.4gb of memory at a playable 40+ fps.
Simply a 5870, 6950, 6870, gtx560ti 384 core gpu does not have the power to give proper framerates when using setting that use more than 1gb of memory.
You need 2 cards to have payable framerates if the settings are turned up to use more than 1gb of memory.