A usefull tool to monitor your server(s): Munin
Gives you provision to easily monitor memory, proc, processes, apache, mysql and much more; and provides you a web interface to see all that at a glance.
And a usefull tutorial, in French: http://www.crashdump.fr/tutoriels/munin-le-top-du-monitoring-3/
(But I am sure there are plenty around in English!)
