• If it says "Preferences," you can be sure you need it.
• Export as much as you can, like all your addresses, all your bookmarks, etc. For iTunes, save all the tunes in your Home folder, plus the files labeled something like "iTunes Library."
• Anything you have installed in your system that you want to keep you should "audit" (I like that... it's a very good description) and back up. Examples include desktop pictures, themes, System Preferences panels, and more.
Hope this helps. Let us know how you come out of this when you're done.
• 2.5 GHz MacBook Pro Core 2 Duo, 4 GB RAM, 320 GB hard drive, runs 10.6.8
• 1.6 GHz iMac G5, 1.5 GB RAM, 120 GB hard drive, runs 10.4.11 (out of commission)
• iPhone 4, 16 GB, iOS 4.3.5