OEM is basically the OS CD and nothing else. No fancy box, manuals, etc. The OS itself is no different then the retail version.
If you're intending on going with a 64 bit OS, I think you'll be better off using Vista. It offers better program compatibility, and since you using all new components. there should be no problems getting Vista compatible drivers.
Besides, you can get Vista Home Basic SP1 64-bit
for much less at $89.99.