Memory used

That's a very low amount, I would not worry until you see it 80% and higher and the swap being heavily used as well.

 

Indeed. Linux memory management is actually designed to use up as much of your memory as possible to speed up frequently accessed readable data by caching it in memory. So, free memory can be very low, but if your cache memory is high, you're usually fine. As dgbaker said, if your swapfile usage is high and cache size is low, you have a problem which will probably lead to memory thrashing.

Here's a thread I posted to explaining memory management a little:
http://forums.cpanel.net/showthread.php?t=33469