the only thing that needs backing up is the GDFS, with a back-up of your gdfs you should be 90% fine, the worst thing that could happen is your erom completey dies which is very rare, you can't back-up the firmware it's not possible but you can always just download the branded firmware, or update with seus that will rebrand your phone, post up a xs++3.1 log and i will tell you what firmware and custpack you need.