3
Perhaps one day, the Xoops.org module repository will be searchable by system requirements - e.g. only show modules that the developer has categorically stated will work in (or at least has been developed using) XOOPS 2.0.16, XOOPS 2.2.5, PHP 5, MySQL 6, register_globals 'off' etc, etc.
Without this information, you have to accept the risk that PHP and MySQL upgrades will probably result in broken modules.
A thread is for life. Not just for Christmas.