Monitoring strategies for infrastructure
If you want monitor several Drupal sites from the same place with alerts, etc... I can say you are not the only one . It's starting to be a business niche, by now filled by:  


Droptor  
New Relic   (Acquia is using it for their
customers)


Both are commercial SaaS approaches, with free limited options to try the service. 

Also there is the Open Source Zabbix monitoring option with its Drupal module http://drupal.org/project/zabbix , but then you should configure and build yourself the server.   

All these options have their Drupal modules to send server and drupal-specific information to the monitor server.  

UPDATE.  Munin has also a specific Drupal Module , with a 2.x branch based on Drush.