The COD4 'Game of the Year Edition' contains an Xbox Live code ('token') which allows you to download extra maps ('Variety Map Pack') . Therefore you'll need to have an Xbox Live account to enter the code, the extra maps are NOT CONTAINED ON THE DISC. The 'GotYE' is an easy way to go about getting the extra content (4 multiplayer maps exclusively for Xbox 360), but you'll probably need a NEW copy because 'used'/'pre-owned' copies will probably have had their token used by the previous owner (unless stated otherwise).
If your 360 is not online then you only need the standard/cheapest copy of COD4 you can find. You can also buy the extra maps separately by using an 'Xbox Live Microsoft Points Card', which you can buy here (I noticed the 2100pt voucher/card is sometimes less than half the price of the 4200pts voucher).
I suppose you need to know how much the separate download costs to be able to judge if the 'GotYE' represents the best value: The Variety Map Pack currently costs 400 MS Points...According to one webpage I just found (11th Sept '08). That equates to £3.23, if using a 2100pt card, which currently cost £16.98.
There is also a 'Call of Duty 4: Modern Warfare Special Edition' (AKA, 'Limited Collector's Edition'), but that just has an artwork book and DVD (See: http://uk.xbox360.ign.com/articles/838/838081p4.html ).
P.S. This game is very highly rated, especially for online play, although the single-player mode is a little short.
P.P.S. You'd be mad not to buy games off ebay instead though. You can get brand new versions of the 'GotYE' for £10. If some rodent kid has badly scratched a used game you've won in an auction there you can have the disc professional resurfaced cheaply by the good people at www.three-r.co.uk. ;)