Call Gamestop and ask how much they'll give you. If they refuse to give you a value over the phone, open up Google Maps, find a Gamestop in a more deserted area and they'll give you a value over the phone. I do it all the time.
This is your life, and it's ending one minute at a time -Tyler Durden
I really doubt you could do it straight up like that. 4GB Slims are what, like $150 new? I imagine they'd give you about half trade-in value, unless they have any "double value toward a system" deals going on.