Try some of these sites, they do have some cheats but most are still hints. If you really want to cheat on a game, get gamesaves and download new levels.
www.boxcheats.com www.xboxcheats.com www.apluscheats.com
This is for Game Saves, you must have the bought the game saves connection for your computer
www.gameshark.com/xbox/
I hope these help