Both GameStop and Target are asking for your old DS systems as trade-ins. They both are happy to give you a nice discount on a new Nintendo 3DS. Take Nintendo’s discount plus your trade-in and you should be able to get a 3DS for a very low price.
Target says that they’ll give you $100 for your DSi XL toward a 3DS. That’s not bad at all! You’re looking at $75 for a DSi and $40 for a DS Lite, so I’d recommend that you take those to…
GameStop is having a promotion this weekend that will give you about $69 in credit when you trade of any of the following: DS Lite, DSi or DSi XL. (Thanks, R0B!) That means you’ll only have to pony up $99 for a shiny new 3DS. There’s a few catches, though. You need to be a PowerUp Rewards Pro member and you’ll have to have a coupon pulled from your online account. The deal is only good from August 12-14.
If you’d rather go to Target, know their promotion is only good for August 12-13.
[Ed: Prices corrected, thanks!]