I put a T7400 Merom in my iMac and it worked great. There is a thread about it somewhere but I am too lazy to link to it. Look for threads I started to find it.
The difficulty of the operation depends on your comfort level of working inside the computer. It can be a bit uncomfortable taking out the logic board to remove the heatsink to access the CPU. As I said in the original thread, it wasn't fun for me.