I first noticed this when i was on the system requirement labs wondering how well my computer could run Age Of Conan. I looked at the video specs and it said that my single Radeon HD3870 512mg (single not the 1Gb X2) has 1.2Gb of Video Memory
I checked the DxDiag and it did show 1276mg of video memory. I can't really look at the card at this time but im pretty sure the company didn't ship the wrong video card (and if they did, yay!)
I tried searching google about this but wondering if anyone else knew the answer?
Edit: Ignore, i found out what it was. It was Shared System Memory. Something new to Vista which assigns memory from RAM to be video exclusive. Interesting tidbit.