Knowing the performance metrics for a system is necessary for identifying resource contention (bottle-neck) and planning for future capacity. Long term monitoring via SNMP and/or tools like Cacti & Nagios are very useful.
dstat both provide very excellent views of system information.
A screenshot from htop showing colour coded CPU and memory information and a process table.
A screenshot from dstat showing pretty output, snapped every second.
Here dstat shows averages collected over 30 second intervals.