Just an FYI:
OEM can only be activated on one computerand once you've done that it's locked to that computer.
As with any version of Windows ...
If you upgrade too much or buy a new computer you'll have to buy a new copy of Windows.
If you upgrade too much or a major piece such as the CPU or HD you have to call in and get a key. If you put it on an entirely different machine you have to assure the boyz in India that you've killed it on the old machine. Once again, as with all XP or greater.
Retail allows you to reuse your copy on upgraded/new computers. (Only one at a time of course.).
Nothing new here, move along.
Also, I'm not 100% sure about this, but I don't think you can use OEM versions to activate 'upgrade' versions of Microsoft software.
There is no such thing as an OEM upgrade. If you have an OEM version of Windows you will get an OEM key. If you have a retail version you will get a retail key. If you get an upgrade you will get an upgrade key. If you add a new device that requires the Windows disk be inserted you will need to insert your proper disk for that machine. If you insert the wrong disk, based on the key info, Windows will let you know.
When I moved my main machine to x64 I installed the XPPro that had been on it, (BTW: that XPPro had been on that machine through 3 chips, several memory steppings and 2 HD's), to a perimeter box. I had to make the call, got the key, and activated the x64 while I had them online. Both installs were OEM.