Your system appears to be mostly free. Linux is very aggressive at caching things in memory before writing them to disk and keeping previously used information in memory in case it's needed again. That's part of why you see the "cached" section at almost 600MB. Also, your swap file isn't being touched - another indicator that your machine is fine with memory.
this is the nice thing about how linux works. In windows they'll tell you the % of resources used, with linux it makes use of all your resources even if they're not being taken up.
