Since you didn't say what your OS is, all we can do is speculate. With a 32 bit system, 4 GB is the maximum that can be addressed, and the system will reserve at least 1/2 GB for itself. If you have integrated graphics, that takes away from available memory too.
Running a Gateway E6500 dual core.
I had 2GB of memory and added 2GB more. My Computer properties only showing 3.25GB.
I swapped the chips and verified that the memory is good. There are 4 slots. Seems like the upper slot only recognizes 256MB regardless which memory is put there.
I suspect a bad slot. Is it that or is 768MB being used by some other part of the hardware?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic