Originally Posted by whyteo
The reason why you have 2GB memory installed, but Vista x64 only sees 1.8GB, is because although you have a x64 CPU, and supporting hardware, one or more hardware devices (probably integrated onto the motherboard) still use 32-bit memory architecture (even if they use x64 drivers), and maps some memory locations during POST, and this makes it unavailable to the system.
My machine has 4GB (4096MB) installed, but only has 3.25GB (3331MB) available for use by the operating system and programs. This is a limitation that is reported to me in the BIOS.
Until all hardware uses pure x64 tech, we're stuck with this problem.