If you have an expansion model(AGP, PCI, or PCI Express) card installed you would disable the onboard video setting found in the bios to see that corrected. The board itself is reserving that much physical memory for use with the onboard.
Don't forget the things like Windows and a number of startups will take away from the total available. If you are not using the onboard video if present disable that or simply reduce the aperature to the default 64mb seen on most boards while using an added in card. See if that sees an improvement there. If you are using the onboard now then you will have to accept the amount taken for the onboard. The memory is working with some of it simply being reserved.
