Hey! Great decision my son LOVES his DS and his been playing for years!
As for games, you already got some great suggestions. I say follow her interests, there are tons of animal games out there, barbie games, racing, puzzle, even pokemon games and you can't go wrong with the imagine series!
My friend's 8 yr old is really into the fashion designing one, and the animal doctor one from Imagine. She also really enjoys Mario Kart and Mario Party.
So take a tour of a local Game Stop store, or Toys R Us and see what they have. A word of advice too, Game Stop sells USED games! Same bang for half the buck, and they take trade ins as credit towards new games.