Hrm, that's lame. In the Oblivion DarNified UI there was an in-game radio button that let you turn off enemy healthbars. I was hoping that would be the same in Fallout 3. If you've got to go rooting through XML files for a solution then that means we still need a mod for this after all. There is an option in Prefabs directory, DUIF3Settings.xml is the file you are looking for. I've just got rid of this nasty little bugger of a bar through editing of that config file.