I recently installed a 4GB DDR2 800Mhz and under Control Panel\All Control Panel Items\System i can see that installed memory 4.0GB (2.94 usable) so i have 1 GB of ram missing !
I had the same exact problem. It is a motherboard limitation. You need to go into your BIOS and enable "Memory Remapping". If I remember correctly, it's in the Northbridge settings.