I have always used OEM. I have an OEM copy of Windows XP, and within the last year have replaced my motherboard twice (with 2 different ones), as well as a couple of other hardware upgrades. Every time I call them to get a new activation code, the only thing they can ask is
"How many computers is your copy of Windows installed on?"
Why is that all they can ask? Simply because MS knows that the whole 1 motherboard per license will never hold up. It is your copy of windows, and MS can only restrict it from being installed on 2 computers at once. MS can't prove that you aren't replacing your motherboard because the last one quit working and they know that.
There have never been any problems, in my experiences, with OEM Windows concerning the EULA, so do not think that you can't upgrade your motherboard if you purchase an OEM.
As for the price difference, I believe that OEM copies must be purchased with hardware.