There is no need to buy a new package if you change the motherboard. You will however not be able to activate Windows. Only thing you need to do is, give Microsoft SA a call, and they'll even ask you if you did a hardware upgrade, and you say yes. They will give you a new activation code, to activate Windows again.
OEM must be prebundled with hardware, i.e. PC, Motherboard, CPU, etc..
Some shops has gone the smart way, where you can buy the OEM version with a Kettle Cord, which is in fact, Computer Hardware, lol -
www.overclockers.co.uk - I bought mine like this ^^ no problem what soever.
Go with the OEM then...