Looking for a cheap way to get your hands on a new Xbox 360 Elite system? The guys at Gamestop are giving out some pretty good discounts if you're willing to part with your old Xbox (and I don't see why you wouldn't).
They'll give you $50 for an old-school Xbox, $200 for a Core Xbox (sans hard drive), and $250 for a Premium Xbox (with hard drive). Not a bad deal at all.
UPDATE: We spoke too soon. Turns out that if you eBay your used Xbox 360 you might fetch a few extra bucks. GameStop has the better deal on the other systems, but you can sell your Premium 360 for $300-$400 on eBay (if you throw in some games). It's not as easy as bringing it to the store, but you'll get an extra Benjamin for going the extra mile.