I finally replace my Core Duo on my iMac from 2006 by a Core 2 Duo 7400 !!! Il works great and the replacement was pretty much easy with patience. I also put a new power supply because the original, a rev.a, seems to cause unpredictable shutdown ... il looks like a component not design by Apple but by my dumb neighbour with his feet ;-)
Again, thx matthewfrey for your precious link. Cheers from France !