You really only need the /include/ost-config.php, but I routinely back up the entire site. This makes backups much smaller and faster but does mean that you would need to re-download the rest of the source files later if you needed to restore. Also going forward to a new version of the code is pretty trivial, but going backwards is... not easy (with out a backup of the old DB).Note: if you are running the attachments: plugin you would need to back up your attachment directoryalso.