Until your next build comes along you can still upgrade things like the cpu. memory, video card, etc. there. Too many items changed(new build by that time anyways) would require a quick phone call to MS to read off the 8 sets of 5 digits to see Windows reactivated.
The main confusion with the term OEM is which type is limited to one machine. The recovery disks provided by HP, Dell, Gateway, and whoever are tied to the one system they come with. On the other hand the difference between retail and OEM for system builders is the lack of the large paperback book on that version and the retail carton.