That was playing Skyrim with the HD texture pack at 2560x1600 with 8xAAA & 8xMSAA. I saw spikes up to 2.4GB during some scenes. I doubt 1.5GB would be sufficient even at 1080p.
You have to divide the memory usage by the number of gpu's you have. So, it looks like you have two 6950's which would be 2233/2=1116MB of vram usage.
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