Other good games that can be found cheap are:
RE:4 (survival horror/action).
Okami (awesome adventure).
Zack & Wiki (point n click puzzler).
Blastworks (shoot em up + construction).
Geometry Wars (shoot em up).
The Godfather and Okami are fantastic games but any of those that i've listed are decent and can be found very cheaply if you shop around (as little as $10 in some cases). There's often good deals on used games too, I bought my son Manhunt 2 ($15) and Red Steel ($5) for Christmas; even though they are not the greatest games they are ok for that price.