As you stated, your operating system is Windows Vista 32-bit. 32-bit operating systems are limited to 4GB max. Part of that 4GB is automatically reserved for various system resources, and thus the reported RAM amount by Windows 32-bit OS will be less than 4GB.
Here are some links that you may find helpful.....
Why doesn't my computer recognize added memory?
Can my system have too much memory?
4GB RAM installed, less than 4GB detected. Why?
In order for your operating system to recognize and utilize the full 6GB you have installed, you will need to upgrade to a 64-bit operating system, such as Windows 7 64-bit edition, Windows Vista 64-bit edition, or even a 64-bit version of Linux.