The one I have used sucessfully is http://www.nagios.org/.
The thing that is nice about it is that it is easy to write your own checks for other devices. It is possible to have it talk to snmp devices too, but I’ve personally never dabbled with that, since it gets complicated very quickly.
Certainly for your size it should be fine. One thing though, its good for alerting you that there is a problem (it can page you or email you), but the reporting tools are not great for historical graphing. For that, we have been using http://www.cacti.net/
Sam.