What files should I backup?

I'm in the slow as molasses process of migrating from Fedora 8 to 11 and want to know what files I should be sure to back up to ensure not only that my mail is saved, but that my folders and identities and contacts are as well.

Any thoughts?  Suggestions?  Links?  Ideas?

The last time I did this it involved a pretty painstaking process of cutting and pasting files by hand and then it still didn't work very well.  I'd love to see a "I'm going to upgrade" button that automates all the necessary stuff and puts it into a format that the new version will read.  :-)