I will help you out here are a few to choose from
but before buying anything i would ask here about which one you should get.
You need an gba flash adapter and a flash card or one of the new adapters that goes into the ds card slot i would suggest the gba adapter because you wont loose it changing our games.
Here is what i use but if you want more space i would get one that uses sd cards
You can do alot more with an adapter then just listen to music and watch movies. You can Run Homebrew Applications like msn and aim messenger progreams and also play backed up games.