There is a lot going on in this chart. It shows terminal server sessions, iops, available ram, cpu, SQL connections, and sql batchs. Together, these are the key counters that really show the health of the system. I don't report network activity because it is not as important in this context.
If you look at this next one for a single file server, I look at different counters. The nice thing about using RRDTool to generate graphs is that you can really build something customized to your environment.
The last one I am going to post is of my Hyper-V cluster. The storage iScsi connections are at the top for each individual node. Across the middle is the aggregate activity of all hosts and guests to the rest of the network. The bottom shows iops on the SAN and available ram for the cluster.
This is still a work in progress but these charts are already providing a lot of value and insight into our servers.