If you can hook your 360 to the internet and you have a 20gig or higher hard drive then I suggest getting xbox live gold... right now on the 1st and 16th of every month they give away a free game. You can get a 3 month for $20 bucks and it will pay for itself with 1 free game.
Since you suggested enjoying killing zombies and Mass effect then I suggest Dead Island - It is another game where you do missions/Level up/skill tree with tons of game play.
Always looking for Kam Chancellor and Rare Marcus Trufant cards. I will buy or trade just message me.