linux - High memory consumption on my VPS

I have a 12GB RAM VPS where I host a a couple of static websites , 2 small Magento powered stores and a couple of WordPress installations - overall nothing exciting and generally low traffic.

I've noticed though that my memory consumption is quite high - kindly have a look at the results of free m below:

             total       used       free     shared    buffers     cached
Mem:      12306384   12137728     168656          0     753360    8629744
-/+ buffers/cache:    2754624    9551760
Swap:      1048564        104    1048460

Also see a screenshot out of Mumin http://s13.postimage.org/q2xewgnef/Screenshot_4.jpg

Now I reckon that 9,5 GB seem to be buffer / cache related - however I find this quite high. Is this something I have to worry about or will they eventually free themselves ? (I've read it somewhere but I am certainly not an expert)

Another note on the side is that Memcached has been installed once and applied to one of my Magento installation - could it be related to it?

Some expert advise would be truly appreciated.