Don't worry it doesn't have anything to do with your card, it is just Microsoft's DxDiag that have issues with Graphic Adapters with 1GB of Dedicated VRAM and 3.8GB Shared RAM (For textures) AMD allocates by default 3822MB of Shared RAM.
Since it is glitched, any value above 4GB turns it back to 0.
Do the math : 1024+3822=4846 4846-4096=750
It gives an "Approx. Allocated Memory : 750 MB" On my card it says 748 MB
That is why all AMD Cards HD4xxx, HD5xxx, HD6xxx have this issue with DxDiag.
Also Microsoft is aware of this issue :
http://support.microsoft.com/kb/2026022
I hope they release shortly a fix since some games uses DxDiag to check GPU compatibility...
Good luck!