The properties pane works on a percentage, not an actual number. If you have 1024mb of RAM, it will get reported as "1.0 Gig of RAM"
If you're getting something less than 1.0, and you have 1 gig of RAM in there, then something's either wrong with the mobo, or the RAM. Update EVERYTHING (BIOS, northbridge, processor, OS, everything!) and see if there's a change. If not, chuck the RAM back and get new RAM.
Or, first run a memcheck, and if that doesn't turn anything up, do the above routine.