My server runs slow: Which parameters must I check?

Hi.

My server runs Fedora Core 2, and it works very slow. Sometimes, when
I type commands via shell, it takes 1-2 seconds to be echoed.

I've just checked free memory ('free -m') and it seems to be ok:
----------------
free -m
             total       used       free     shared    buffers     cached
Mem:          8104       7642        461          0        220       3979
-/+ buffers/cache:       3443       4661
Swap:         4094          0       4094
------------------------

If I check "ps -ef" I don't see any weird process and with "top" the
most memory-wasting process is 'mysqld' (0.8%).

Which extra parameters might I check? Thank you very much.