I should have added that I have 50 Switches and Routers to monitor. With that in mind what would be the best way?

You can use SNMP based products such as Nagios (open source) or Solarwinds NPM (commercial) for monitoring and long term trending of your network devices.

http://www.nagios.org/

http://www.solarwinds.com/products/orion/

Hope that helps

Using show proc, show mem, show interface what would be the threshold between having a problem and not having one?

You need to baseline your setup.

For some of my boxes 80% CPU load is pretty normal as they are doing heavy duty tasks.

However if I see it spiking to 85-90% I would know some recent changes have upset the box.

HTH

Sam