Why does 4 GB ram only show 3 GB?

Answer

If you are using two memory modules in your computer, make sure both are inserted properly. If you remove one and the system still works, you know which one is not inserted properly.

If you are using a 256 MB module, you may have purchased a low quality module made using "low density" chips. These modules use slower chips to produce a memory module. These are incompatible with most chipsets, and are not standards-compliant. They usually only work well in low-end systems.