All good OS's try to use all the RAM availible (within certain limits of course). My RAM is almost always at 80% usage and above even with just a couple of lightweight applications running.
I was talking about "Commit charge" which is the memory actually used by the processes. You can see it by enabling the Commit size column in task manager. Which doesn't even include all the memory process actually uses. Check this for details on Windows if you're interested
http://blogs.technet.com/b/markrussinovich/archive/2008/11/1...
