LAN/WAN Monitoring and Trending

I’m trying to build more statistics gathering and reporting & alerting mechanisms into my LAN/WAN environment. I want to use SNMP for most of this functionality. Any good sites out there with SNMP how-to’s, strategies, etc.?

I would like to know:
> When servers/services go down
> bandwidth utilization over WAN links
> When network traffic levels reach thresholds
> I would like to be able to automate the gathering of this information and development of daily, weekly, monthly reports from this data.

Any information, suggestions would be greatly appreciated.