That's how I check true free memory available:

  root@joe-pc:~# free -m
               total       used       free     shared      buffers     cached
  Mem:         32057      30347       1710          0         1312      21568
  -/+ buffers/cache:       7466      24591
  Swap:            0          0          0
  root@joe-pc:~# sync; echo 3 > /proc/sys/vm/drop_caches
  root@joe-pc:~# free -m
               total       used       free     shared      buffers     cached
  Mem:         32057       5628      26429          0            4        891
  -/+ buffers/cache:       4732      27325
  Swap:            0          0          0
  root@joe-pc:~#