Assuming that you have installed the FULL
driver for the card from nvidias website and not just the windows update
driver, the answer may lie in you BIOS settings.
I assume that you are on a 64 bit version of windows?
I can't point anything out specifically, but there may be settings in your BIOS WRT the video card that may limit the amount of ram is uses (AGP Aperture, etc). If you were to stick that card in to a computer with 32 bit windows you would have NOTHING left to run your OS/programs unless memory aprturing (or whatever it might be called in your BIOS) was enabled.