You can select a playlist as your accompanying music in iPhoto, so you can have as many songs as you wish. It's funny how when you only have one song in a loop for a slide show, you end up *hating* the track by the time it plays for its third time.
You can certainly do the above in iPhoto 6 which is what I have. I assume the same is true for iPhoto 8 or whatever we are now at.