OS X :: Increasing RAM Usage?

I'm new to OS X and just had a quick question about the way that it uses memory.

I have the istat widget on my dashboard, and I've been monitoring the amount of free and used RAM. After I reboot the system, only about 20 percent of my memory is being used between the OS and the programs I normally use (adium, safari, itunes, etc.) Over the course of a couple days, the amount of used ram increases to about 50 percent, even though I'm just using the same programs.So, I'm just wondering how exactly the operating system deals with used and free RAM. Once the system uses ram for something, will that ram be reported as used until I reboot? Why does used ram continue to increase even though I'm doing the exact same things?

I'd just appreciate if someone could enlighten me on how the system uses RAM and whether I need to reboot every few days for optimal performance. I know it's still only using 50 percent (i have 4 gigs) - I'm not afraid of running out, I'm just wondering for my own knowledge.