What does installed memory 4 00GB (3 00GB usable) Mean?

Are you running on onboard graphics.? In that case, 1GB of your system memory may have been reserved by the onboard video chipset to be used as video ram.. Thus the resulting amount of system memory left for the OS to use will be 3GB..

Thanks,
I think that might be it. It says my
I have a GeForce Go 7950 GTX
total available Graphics Memory is 1791 MB
Dedicated Video Mem 512 MB
System Video Memory 0 MB
Shared system Memory is 1279 MB