I installed the Windows Vista Ultimate final over the weekend and here are some notes on what I did before doing a fresh install.

Run the Windows Vista Analyser to determine how well your pc can handle Vista.  Don't expect it to catch everything, it will not.

Do a full bootdrive backup. No seriously, do it! I recommend Acronis True Image.

Decrypt all files encrypted by the os (or if you have the know how, export your certificates, etc.)  Your encrypted files will not be readable in the new OS since the user's SID will be different.  (If you are upgrading and not doing a fresh install, you probably don't need to worry about this.)  If you backed up your encrypted files with Windows XP/95's backup tool before you decrypted them you will still not be able to read them as the backup actually backs them up as encrypted files.

During the install, I chose to format the boot drive since I had done a full backup.  I have never fully trusted Windows upgrades, so I always do a full install.