One of the best system monitoring tools out there is a product called Xymon.

This is a open source product and is highly customizable. Out of the box it comes with a large number monitoring variables (uptime, cpu, memory, disk, network) and also built in graphing with RRD.

I have written a couple of scripts for monitoring Ingres installations around transaction log file usage and monitoring the DMF cache size.

I would recommend that if you have limited system monitoring in your organization then you should investigate Xymon (formally called Hobbit).

For more information check the Xymon home page here:

http://www.xymon.com/

There is also a wiki book page:

http://en.wikibooks.org/wiki/System_Monitoring_with_Xymon