Bandwidth utilization can be monitored via MRTG as can any other SNMP OID (like fan speeds, disk utilization, etc) and outputs to daily, weekly, monthly, and yearly graphs per OID. The one thing that might not suit your needs is that it polls every 5 minutes, thus it's not real-time.
Another option is WhatsUp by IPSwitch. You can use it for polling and notifications, including SNMP polling and notification is a threshold is crossed. Notification possibilites include SMTP, TAP paging, Call a phone number and delivering a pre recorded message, and more.