Having set up lots of monitoring over the years using Nagios and its predecessor Netsaint as well as HP OpenView and Sun Net Monitor I can tell you that figuring out what to monitor is always an excersise that needs to be well thought out.

One thing I'd caution against is monitoring too much. Anything you run against a system is going to have some form of penalty however slight that might be. If you have a lot of slight penalties you can cause a death of a thousand scratches to what you are trying to monitor. Sort of an extreme example of Heisenburgh Uncertainty where you are affecting what you are trying to measure.

How I normally select what to monitor is to first determine what is important to monitor. That whole list you have, however impressive may be, may not be all items that are important to monitor. Start with the basics.

• What applications am I running
• What will the user see if it goes down?
• Is the box up/down?
• Is it usable?

Then you build from there.

Having said all that... I'll just say this: K.I.S.S.