lammert - 1:24 pm on Jun 14, 2010 (gmt 0)
First of all the command free -m This will display the current memory usage. If you see a significant amount of memory used as swap, then RAM size could be your problem.
The next command is uptime. This gives three load average values. If these values are high (anything above twice the amount of CPU cores is suspicious), it is an indication that many processes are waiting in the run queue to be started. This doesn't necessarily mean that your CPU is overloaded. It can also indicate that processes are waiting in the queue because of an hard disk or network I/O activity which must be completed.
These two commands should be typically be used when the server is at normal load and hasn't been restarted for a while. Some other commands which give more detailed information are top and iostat, but the need for them depends on the outcome of free and uptime.