I use
SERVERS ALIVE! to monitor 5 boxes, and I am very happy with it!
Very configurable -- hell, it sends me an SMS, emails me and my 2 techs, and even ICQs me/us when there is a problem ... it has scheduling, so you can set it to notify different people at different times!
But running the software remotely on your cable modem will give you more false alarms than you care for ... you will want to put the software ON THE PHYSICAL SERVER(s) -- set it on at least 2 of them, and have them ping the others as part of the rules to determine if there is a problem ... if you set it on one machine, and that machine happens to go down, none of your other network is being monitored ... lol
Hope this helps!
FF