"I have been meaning to look into Nagios. I have some 16 machines under my care now.."

I'd just like to throw in a plug for Big Brother. I've been using it for several years now, and currently we're running about 15 different tests on 600+ hosts - and it never misses a beat.

It's extremely easy to setup and configure, and also quite easy to customise to specific needs. I can't say if it's better or worse than Nagios, as I've not used that. Just offering an alternative :)

Cheers,
Darren :)