I am sorry but I had the understanding that OEM stood for "Original Equipment Manufacturer" meaning it would be a copy that ships with say an already manufactured PC. I was also under the impression that a new retail version of an OS was made for building new PC's or upgrade other PC's.
OEM does stand for "Original Equipment Manufacturer." When you buy lets say a DELL yo will have an OEM copy installed. The reason that the full retail versions are so expensive is because you get 32 and 64 bit together. OEMs are just one ore the other and are a cheaper way for people like me to build computers because I know exactly what version I need. Think of it this way. DELL uses OEMs and sells to you. I use an OEM and sell to my friend. Also if I build a PC for myself then I can use an OEM and save money.
While the retail upgrade version is useful for the general market to do just that, the full version is mostly a way to get more money from people that don't have the tech savvy, or whatever, to know to get an OEM copy.