[wlug] Memory usage

I'm a newbie using RH9 In system monitor is see my memory usage is 303Mb, total 312Mb. CPU usage is between 2% - 5% with no significant processes running. 1. Why is so much memory being used? 2. What could be using it? 3. If a process was running, would the memory become full and start using swap space? Any light on these points would be appreciated. Cheers Gun Caundle