There are some great apps for doing what you are looking for. Personally, I like MRTG. There is a wealth of information on the internet regarding MRTG. Basically, it uses SNMP traps to monitor loads of SNMP enabled devices.
www.google.com/search?hl=en&q=mrtg
Another one that I have been itching to test is Zabbix.
www.zabbix.com/
It uses similar methods to capture data but also has a client to support non-snmp network devices.
Drew
