Vista and DX10 can virtualize the graphics memory so system memory can be used for graphics operations
as well as buffers to transfer from / to the graphics card, et. al.
I doubt that that amount of memory is "really" being used to the extent that it is unavailable for more important uses
when it is needed for other programs. Try running a couple of memory heavy programs and see if they successfully
work despite that memory area (maybe it is a 'virtual' area and not a physical one) or if the memory area is visibly
shrunk / reduced when there are other memory demands.