What to monitor on the network and how often

I’ve recently moved into the performance monitoring area and after poking about on techrepublic I reckon that in terms of best practise the good things to measure on your IP network are:
1. Network Availability
2. Router CPU/Memory
3. WAN bandwidth utilization
4. Switch uplink utilization
5. Firewall availability
6. Firewall CPU/Memory
7. Syslog files from all networking devices.

However, what I don’t know is how often to poll on, for instance, the first 4 items. Options might be to poll every 15 minutes or every 5 minutes. Would the accuracy of the data be significantly affected? I’m imagining that the peaks would merely be smoothed out a touch.