Effective Network Monitoring

General architecture…small class C…about six switches…couple of routers…hardware firewall…three Win 2000 servers housing website, cc server, email server…Plan to monitopr this via snmp traps, but monitoring software must run as a service…i.e.I don’t need to be logged on…must be able to send a page while running as service…just a gimme, must be remotely accessible (either dial-up or vpn)…will be monitoring for bandwidth usage, port scans, smtp traffic, and other detriments tomy company…Anybody have any thoughts…all of the vendors I have spoken with claim to run as a service in their next release…However, I’ve made that song and dance before myself.
Thanks