Once installed, MS will lock down all the registration details
to the hardware. So if you try to activate it and your hardware
specs is different, it will not work, and you will get the error as
mentioned.
Just go Hardwarezone and look at the price guides. 2007 is much
more expensive than 2003.
Box and OEM not much of difference, just the pricing. If you want a
cheaper one... get the student/teacher edition or home/student
edition. You have to prove that you are a student or teacher to get
it...