Usage refers to how much is actively being used.... just because you have 6GB doesn't mean it's all needed all the time. A system using all it's RAM at one time is usually bogged down and laggy.
Using about half the available RAM means you have room to open another program or process without slowing down.
Now, if you're seeing only 3.16GB installed TOTAL, then there's a chance one of your DIMMs is going or gone, or you have a 32 bit OS (XP), and it's capping you.