Regarding the OEM version: If you use the OEM version to upgrade, you are responsible for providing your own technical support or paying someone for support. Microsoft will not help you. For most people this will not be an issue, but it is something to be aware of.
Another crazy tidbit of info about the OEM is that the Microsoft licensing agreement states that you cannot install it on your own computer. You should only use it on a computer that you sell to another individual. Therefore, you need to install the OEM, sell the computer to a friend for a penny and have them give it back to you