As I understand it, an OEM key is locked to the motherboard. It doesn't matter whether your copy of Windows 7 is preinstalled by the Original Equipment Manufacturer, or a RTM release put on by yourself.
The only thing to note is a key for Home Premium will only activate Home Premium, Pro for Pro, etc. Also note that the OEM key is good for either x86 or x64.
As for your post #12 - I can only say that I have done this and it works, OEM --> RTM & Spanish --> English. Be advised you do not need to do the ei.cfg tool thing if you are downloading your own version of Windows - this is for unlocking all image versions of Windows 7 from the install ISO.