um, i don't think thats shared memory, but total available for the card.
your card has 640, plus the 128 meg apeture window.
are you looking at the used memory vs free memory?
- Dedicated system memory 0 MB means no RAM is reserved (i.e. you don't have
a graphics card integrated into the mother board).
- Shared system memory is 766 = 640 + AGP aperture size. Shared doesn't mean
used! Games will use this mem via AGP aperture size