A computer which is running Windows 7, the usable memory (RAM) of this computer may be less than the installed memory. Suppose, a 32-bit version of Windows 7 may report which there is only 3.5 GB of usable system memory on a computer that has 4 GB of memory installed. The amounts of usable memory are not exact amounts. Basically usable memory is a calculated amount of the total physical memory minus hardware reserved memory.

If you want to view the installed memory and the usable memory in Windows 7, then you have to follow these steps. At first Click on the Start button, and then right-click on the Computer and then click Properties. Now, View the Installed memory (RAM) value under System. Now, if it displays 4 GB (3.5 GB usable), this means that you have 3.5 GB of usable memory out of 4 GB of installed memory.

Cause of these problems

This is expected behavior on computers which are running Windows 7. The devices that are installed in the computer along with the memory which is reserved for those devices. The ability of the motherboard can handle the memory and The System BIOS version and settings. The version of Windows 7 which is installed and control other system settings.

If you have a video card which has 256 MB of on-board memory, that memory can must be mapped within the first 4 GB of address space. If 4 GB of system memory is already installed, then part of that address space must be reserved by the graphics memory mapping. The graphics memory mapping can overwrite a part of the system memory. These conditions reduce the total amount of system memory which is available to the operating system. There are several additional situations which could cause the usable RAM to be less than expected. These issues and possible solutions are like,

System configuration settings
This problem may occur when the Maximum memory option is selected incorrectly. If you want to fix this, then you have to follow these steps. At first you have to click on the Start button and then type msconfig in the Search programs and files box, and then click msconfig in the Programs list.
In the System Configuration window, you have to click on advanced options on the Boot tab. Now click to clear the Maximum memory check box, and then click OK. And finally you have to restart the computer.

System BIOS
The problem may occur when the system BIOS is outdated. If you have an older computer, then the system may be unable to access the entire installed RAM. In this situation you have to update the system BIOS to the latest version. If you want to update the BIOS on your computer, then visit the Web site of your computer manufacturer for download the BIOS update. Or, you can contact with your computer manufacturer for help.

It is better that if you buy graphics cards with dedicated memory, so if the memory size is less than that is being shared from RAM. The reason is that, in shared memory the memory buses might be m occupied by the graphics system which can slow the system down. In dedicated memory can avoid this problem because the RAM is not touched for graphics rendering in which the bus is free for your processes.