Kapvin; I am no expert on this at all, but I understand that an OEM copy is "sold for use with one PC", either the one you bought it with, or the one you just built and are supposed to stick the key to. That's why MS started putting the ky stickers
on the machines. A non-OEM licence, is yours to load on one machine at a time for ever more into the future.
I hear not only what you say about the motherboard upgrades, but would add that I am in a similar position, with an OEM XP now running on it's second motherboard. It was installed and activated by the maintenance company for Compaq, even though it is not a Compaq board. I suspect that MS activates stuff for them daily; they are probably on first-name terms! I shall certainly give it a try, before spending my money.
theironhorse... thanks, and I will not try to pretend that all the commercial software on my machine was actually licensed
to or paid for
by me. That would be hypocritical

. My copy of W-XP
is legal, albeit OEM: if MS activate it for me, I shall wander off whistling and use the the thing --- otherwise, I am actually prepared to pay some money to the beast on this!
Between you and me, (Shhhh!) One of my favourite pieces of software was not paid for, or not, at least, by me. You know, I'd be prepared to pay for the up-to-date version, except I hate what Adobe (maybe gave myself away there) have done with it --- I downloaded it, in its enormity, to try. Google searches tell me that nobody has made my old version work on Vista or 7. If it wasn't for this one thing, I'd probably bite the bullet and give 7 a try.
XP mode would be very interesting to play with, but when it comes to hardware stuff like communicating reliably with sound cards, I understand there are doubts.