Do it all in AE7.
Remove the green screen of each clip using the Keylight plugin (you may need the Pro version for it though)
All you then need to do is animate the position of each clip along the timeline. You'll get better results by using the motion tracking tool each time on the layer below.
For extra credit you can then go back in an d apply faux shadows to give the effect some realism.
All you need then is some crappy song to put over the whole thing
