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I want to say there would be an issue with the front side bus (100mhz vs. 133 mhz). I'm no expert though so I might not have a clue.
 
Good point. It might run slow (dual 400) or not at all.

I've run the uni-north-checker and it says I have a rev 7, so a dual upgrade would work, but the bus speed is probably a deal-breaker.
 
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