I assume you're talking about the operating system version? It really doesn't matter, you're supposed to buy the retail version, which can be installed on multiple different computers (though not simultaneously) and is more expensive, but you can get away with buying the OEM version. The OEM version is cheaper and can only be installed on one computer (to be more specific, it is linked to the MAC address of the motherboard). Though if you call up microsoft they might be willing to make an exception for you if you want to switch your mobo.
Hope I helped