Memory upgrade?

Some of that memory is mapped to your video card.
Some more is mapped to the various I/O devices, like the USB and sound adaptors, as well as the keyboard.
Then, there is the ROM of the BIOS.

It all ads up to roughly 1 Gb, the video part been, by far, the bigest contributor.

Unless you have a 64 bits CPU and a 64 bits OS, you can't use that part. The CPU, OS and drivers DO use it, but the applications can't.

Electro
October 2011