The simplest way that I can think of would be to uninstall iPhoto 6, and then re-install iPhoto 5. Before you uninstall anything, move your iPhoto library to another location, and drag the folders (full of your pictures) into iPhoto 5 once it has been installed. It will import your photos. I'm not sure if the library itself will work with version 5, because it is a newer version, so dragging all your photos back into iPhoto will be your safest bet.
As far as your settings, you will lose those.