It sounds like you might want an SNMP monitoring tool. I'm familiar with InterMapper, which has alerts and so forth (email, I believe, but they might do SMS) and a very nice user interface. There are plenty of open-source SNMP tools, though, such as net-SNMP that can be made to do what you want with a little tinkering.
My own background is in NetFlow monitoring, which can be made to do what you want, but if I were in your shoes I'd look into SNMP first.