trade in value...

hey guys merry christmas!

i got my jasper from my girlfriend today and im very happy with it, its nice to have a dvd drive that doesnt sound like a car.

but anyways, what i was wondering is what is the trade in value of a 20gb premium? i know there was a thread somewhere about this but i couldnt find it.

basically im thinking of trading in my old one and using some of the money towards a 120gb hdd i notice with my new xbox, i can get a refurbed 20gb and 3months xbox live for 20 quid, do you think its worth buying that, keeping the xbox live and trading that hdd in aswell? reason i ask is i need to be able to transfer the stuff from my current onto the new one but im not sure they'll let me do it as i trade it in and im not sure they'll accept the xbox without its 20gb hdd.

whats the best way of doing this then?:)