From my own experience I would recommend some good and "smart" software that can distinguish between server down and PING time out for e.g.
Otherwise you will get server down e.g. every night when you do your backup over the network and you have high load on the local network?
There are quite a few commercial products out there. Personally I use two of them. IP Monitor from MediaHouse and MOM (Microsoft Operations Manager).
Both products can alter when the server is down. You can pageor send mail to different people ? depends on the server or the problem (link down, server down, ...). You can also set the time when they send mail (e.g. 9-5) but not at night?
MOM is an Enterprise solution and is quite expensive ?
The only problems are one point of failure devices such as mail servers (no mail server no alert), main router, one ISP connection, etc?
Regards, Mike