Just about to upgrade to Snow Leopard (from Tiger). I have backed up all of my important files using Dropbox. Is this sufficient if I want to just upgrade (rather than clean install), or is it necessary to get hold of an external hard drive and back everything up? I presume most of my applications and software should be downloadable in new formats if the current versions are not compatible with Snow Leopard. If all fails, will I be able to uninstall Snow Leopard and just revert to Tiger again?