Do you have a separate video card or is this on your MB. If the later, your RAM might be shared with video and your system will not report that portion.
I've got a 1GB HP PAV Processor and am supposed to have 128MB of SDRAM only 92 MB show up on the Performance Tab. Any ideas where my memory has gotten to? I've tried shutting down down drivers one at a time to find a change.....no dice.