Hi, I have 4GB of RAM installed, but Windows tells me that only 3GB is usable. This seems rather odd, since I'm running a x64 system, leading me to think that there may be a problem with the new RAM I bought. however, when I swapped the 1GB module and tried 2 different 512MB modules, Windows displayed "3.5GB (3GB Usable)". I'm no expert, but I think it could be a limitation by either the Asus P5LP-LE motherboard (even though HP states that it can take 4GB), the MSi GT440 graphics card (even though it has its own DDR3 RAM), or the Pentium D 820.