Does you system have the 32bit version (x86) or the 64 bit version (x64) of Windows 7? If it has the 32bit version then you will not see more memory than what you already see. The limit for 32bit operating systems is 4GB but some of this memory is reserved by the system's devices that is why you see less than 4. To see all 6GB you will have to install Windows 7 x64.