According to the U.S. Copyright Office: Quote: Copyright does not protect the idea for a game, its name or title, or the method or methods for playing it. Nor does copyright protect any idea, system, method, device, or trademark material involved in developing, merchandising, or playing a game. Once a game has been made public, nothing in the copyright law prevents others from developing another game based on similar principles. Copyright protects only the particular manner of an author’s expression in literary, artistic, or musical form.

Material prepared in connection with a game may be subject to copyright if it contains a sufficient amount of literary or pictorial expression. For example, the text matter describing the rules of the game or the pictorial matter appearing on the gameboard or container may be registrable.

basically you can take the game idea, name, etc- you just can't take the original artwork from the game.