bb2k, interesting project...
I think someone has mentioned it earlier, but I would think with today's athlons xp, cooling the cpu backside would result in very minimal. and keeping those tubes cleared...
I found this thermal image of the backside an epox 8rda+ mobo interesting. See how much heat the mosfets are creating and how that is effecting the heat near/behind the cpu. If you can reduce the heat produced by those mosfets, I would think you would have a greater impact on cpu temps.
Source:
8rda.com