OEM is meant to stay on the PC (MOBO+CPU+HDD are the main bits that make up the build of the PC). Retail is transferable, only 1 copy installed at a time.
From this site and others I have previously read that you can upgrade an OEM version with upgrades, and you can do a clean install of the new OS with the upgrade media. But only on retail versions. So if you have a retail version that is eligible you can use it for what you want.
The only stipulation was you had to keep the bits, so 32-32 and 64-64. But I think MS changed their stance on that due to the wide adoption of 64 bit, and wide use of 32 bit for the upgrade path.