I think the full retail version comes with a certain amount of free online and phone support, while the OEM versions do not, because the place you bought the computer is usually responsible for that kind of support.
The OEM version is the version that is installed on a preassembled computer like Dell, Gateway, HP, Compaq, Emachines, ETC., Microsoft sells the OEM version at a reduced rate because its to be bought with hardware, (IE Usually a Computer but sometimes with a Motherboard), and then some companies bend the rules a little, to help out the little guy and make money, and sell the OEM version with, miniscule hardware, say a screw or mousepad, and its legal because hardware was never really defined properly, but thats besides the point.
The OEM version is all you need, Updates from Microsoft are available to either version so you don't have to worry about that, most everyone here is using the OEM versions, so you're in good company, trust me don't worry about it, go ahead and get the OEM.
Thats all I've ever run!
<A HREF="http://forums.btvillarin.com/index.php?act=ST&f=41&t=2541" target="_new">My Rig</A>