These tools are great for looking at what's happening now if you're logged into the server.
They're complemented by monitoring products like:
Self hosted:
- Nagios (already mentioned in the post)
- Cacti / Munin
Hosted:
- http://www.serverdensity.com (tool my company produces)
- http://www.cloudkick.com (monitoring + cloud infrastructure management)
these give you similar metrics plus various other things like alerting, graphs, mobile apps, etc.