Knowing what's happening on your servers is important, that's monitoring. Knowing what happened on your server is establishing a baseline. You need to do both. I really enjoyed this blog post by Ted Krueger (blog|twitter). It's not enough to know what happened in the last hour or yesterday, you need to compare today to last week, especially if you released software this weekend. You need to compare today to 30 days ago in order to begin to establish future projections. How your data has changed over 30 days is a great indicator how it's going to change for the next 30. No, it's not perfect, but predicting the future is not exactly a science, just ask your local weatherman.

Red Gate's SQL Monitor can show you the last week, the last 30 days, the last year, or all data you've collected (if you choose to keep a year's worth of data or more, please have PLENTY of storage standing by). You have a lot of choice and control here over how much data you store. Here's the configuration window showing how you can set this up:

This is for version 2.3 of SQL Monitor, so if you're running an older version, you might want to update.

The key point is, a baseline simply represents a moment in time in your server. The ability to compare now to then is what you're looking for in order to really have a useful baseline as Ted lays out so well in his post.