Typing "top" will show you a list of all processes running, with a rundown of how much memory and cpu is being used, and by what program. typing "uptime" will tell you how long your server has been running since its last reboot
System information as of Sun Apr 4 23:49:36 CEST 2010
System load: 0.01 Memory usage: 49% Processes: 131
Usage of /: 2.3% of 75.49GB Swap usage: 2% Users logged in: 0