If I remember right, each type of core hardware device is assigned a weight value in terms of relevancy concerning a "new computer" and Windows Activation. GPU, CPU, RAM, NIC, etc, etc... Of all of them, and as odd as it sounds, I seem to recall the NIC having the largest "weight" in terms of hardware changes.
The motherboard itself doesn't register as a device in windows. It does have several onboard components that do though, including the NIC if you're using integrated networking. That's why changing the motherboard usually means having to use a "new" activation. In theory, if a new motherboard had most of the same onboard components as the old it might not require re-activation but Windows might be looking at unique serial numbers (not sure) and not just hardware device IDs too.
-dolynick
& 
. There goes another W7 activation.
