Please post the following if you are concerned
Remember that Linux will attempt to use any free memory for caching disk blocks. The following is from my mini-itx system with 2GB RAM
Code:
total used free shared buffers cached
Mem: 2017 1916 101 0 177 759
-/+ buffers/cache: 978 1038
Swap: 4031 0 4031
The free buffers/cache value of 1038 is way below the total of 2017 so even though everything looks used a high proportion of the memory has been allocated to disk block buffering.