Originally posted by tripleione
They are not compatible. Willamet and Northwood have different number of pins on the CPU. Thus, if you try to fit a Northwood into a Socket 423-based board it will not work.
You could buy a 423->478 adapter, but it's best just to buy a new motherboard.
Originally posted by NoisySilence
There are Willamette CPUs in the S478 format, it's not a problem with the pin count.