Assault Cube

Assault Cube is a fun Multi-Player/Single-Player game. It includes good maps and game modes like Team Deathmatch, Deathmatch, and One Shot One Kill. Also, if you are a creative gamer, you can spend the time making your own map in its very versatile map editor. It's graphics are good and it runs nicely, except when you first start a map. When the map is done loading, it will run at about 5FPS, but then it will get going to a good, regular speed. The game has online and bots mode, which is great! There are 4 guns, an assault rifle, shotgun, SMG, and sniper. In older versions of the game, it is called "Action Cube."

Download from their website: Assault Cube
To get a version working for the PPC, look in the software archive and experiment around.