I was finding it very weird that only 700 MB of my 4 GB RAM memory was free when I was only running Chrome… After reading a lot of forums about memory management and chrome and all, I found out that Linux uses my unused RAM for buffers and cache. During my search, I installed a quite cute version of top, called htop. It has colors and a chart =)
But an easy way to see how much of your RAM memory is being used by buffers and cache is with the command free. In my case, the output was:

             total    used    free  shared  buffers   cached
Mem:       3909684 3201664  708020       0   180988  1648576
-/+ buffers/cache: 1372100 2537584

If I check the second line, I can see that the used memory is actually about 1.3 GB (still a lot, in my opinion), which is exactly the used memory from the line above minus the buffers and cache parts. I read somewhere that Linux has a lazy deallocation, so I don’t expect to see a free RAM memory anytime…