If you are not going to restore system database make sure you record the sql server logins (windows and sql authentication) and their permissions, as these will need to be reinstated. Make sure you know the passwords for the sql authentication logins. SQL jobs will need to be recreated, so script them out and save the script. record the server properties that are currently set so you know what the memory configurations etc are.

Snapshot the machine if it is virtual before you do anything.