You should definitely buy Meteos, the two games bobman37 suggested (and, if you can find it, Tactics Ogre: The Knight of Lodis), and SPAC3 INVADERS EXTR3ME. You might also enjoy Mario Hoops 3 on 3, Namco Museum (or Namco Museum DS, since it has Pac-Man VS and some awesome games the GBA version is lacking), Arcade Advanced (a Konami collection for the GBA containing Frogger, Rush 'n' Attack, Gyruss, Scramble, Time Pilot, and Yie Ar Kung Fu; a larger collection also exists for the DS, and contains some of the aforementioned games, plus Twinbee, Gradius, Contra, and a few others), the Mega Man Zero and ZX games (avoid all other Mega Man games on the GBA and the DS, unless you want to spend a bit of money on one of the versions of Mega Man Battle Network 3, which was the only really good game in that series), Astro Boy: Omega Factor, and Gunstar Super Heroes.