I use System Monitor to check free RAM:

But free don't say the same:
tuankiet65@UbuntuPC:~$ free
total used free shared buffers cached
Mem: 8092368 7666932 425436 0 444560 3309676
-/+ buffers/cache: 3912696 4179672
Swap: 8370172 537836 7832336
So, which one is correct? System Monitor or free?