Is there any difference between the logic boards of the 7500, 8500, 7300 and 7600? Or, indeed, were there revisions of the logic boards within the same series (e.g., the 8500) as the processor speed improved, etc.?

Presumably the (original, non-Kansas) 8600 is a different animal, as is the 9500 and the 7200, but what about the rest? Video system etc. etc. all identical or different?

I hear lots about the boards being interchangeable, but am not sure just how far that piece of wisdom really goes.

Here's another way of putting the question: if swapping logic boards between early pci macs, which is the best?