Well, GameStop's current trade-in value for a GBA SP is currently $45. This is the best I have seen, with EB Games taking them for $30. Take that and put it toward a NDS. But, if you planned on doing it, I would suggest doing so quickly, as I forsee a jump in GBA trade-ins in anticipation for the NDS, which would in-turn lower its value. Also, I suspect a possible GBA SP price drop after the NDS's release, which would lower its value further.