[quote name='DetroitSportsFan']I wonder what Gamestop will give for a used 20GB.
I'm thinking of selling mine to get a 40GB. I never use the backward compatibility.[/quote]
Hell, if you find Gamestop's offer to be less palatable than you originally thought it would be, then list it on CAG. I'm sure SOMEONE would buy it from you. Hell, I'd probably do $50 or more over their offer, depending on what it is.
So, keep us informed.
EDIT: I just checked the CAGwiki and a 60gb model was getting a meager $250 in trade value @ GS as recently as 12/29/07, so I'd imagine that the 20gb model is about half that or MAYBE a little more($175 would be my estimate).
So again, like someone else said, keep the system since I doubt it's worth enough to make it a near even swap for a 40gb.