Incomprehensible RAM usage

Hello everyone. So I've been trying to keep track of how my 2ghz unibody Macbook uses its RAM, and I've been curious as to why after some time it reports that I have as little as 30 MB of free RAM left... It obviously reports that the rest is in use, but the numbers just don't add up to 1.9GB of used up RAM. What is my computer doing that is not being reported, and how do I get it to stop? It has made my machine noticeably slower.

I've posted a screen shot to illustrate.

It may help to know that I had been a Windows user for years. In Windows we can launch msconfig.exe to manage which programs get to run in the background and at start up, and from there we could decide to shut down programs that check, for example, for RealPlayer updates and other completely unnecessary executables. Can someone give me a run down of what I can safely eliminate from my processes in my Macbook, and therefore keep more RAM free at all times?

Many thanks,
bahen

 

But the RAM usage still doesn't add up...