I have avoided doing this myself because of everything that I've read previously required using several addons that included fubar. I hate fubar.
Today, I gave
http://addons.us.to/addon/theorycraft a shot and it has a lot of toys to play with. Once you've loaded type /tc and another window will come up with all the configuration information. There are three tabs on the TheoryCraft window.
I'm currently on my warlock and have added a bunch of info to each of my spell tooltips about their DPS, crit chance, etc. There is also an overall tab that gives me specific info about the character that is not included on the default character page.
I've only tinkered with it for a few minutes so far, and I've gotten a lot of info that is not included in the default.