Take a look at my comments in this answer:
Consider whether these best practices fit you and whether Nagios is a good choice.
If you are looking for background, consider the Nagios Web site
and the Linux Magazine overview article (still available for free),
or a site dedicated to overviews of the various systems available http://www.monitortools.com/cat_networksystem/
Whether it is a better or worse solution for you to consider really depends on your network and your objectives. And your best practices.
This was first published in June 2003