Put on the headphones and try to think of as many ways to call someone gay as possible!
The two games you already have will keep you busy for awhile. COD4 was probably the best action game experience I had this year. A very well made, exciting game.
Rock band is a game you should look in to if you're at all in to the genre. Great fun, particularly if you're a fan of the music included in the game. Even if you're not, the game might make you a fan.
Mass Effect I liked, but I'm not sure it's one of those games that everyone would like. If you've ever played knights of the old republic then you have an idea of what its like.
If you're at all in to classic western rpg's, there's a fantasy rpg called Elder Scrolls: Oblivion. (Essentially a Dungeons and Dragons type of world) Basically you make your character, pick a class, then get dumped in to a living world and can do whatever you want. There's a main quest to follow, which you could start as soon as you start the game at level 1. Or you could do what I did and start the main quest when you're already a world renown hero and gladiator arena champion with a house in every major city in the world. Mini quests abound, and there are a bunch of organizations you can join with their own quests, like the fighters guild and the brotherhood of assassins.
If you ever wanted to play a futuristic grand theft auto with a super enhanced cyborg, then Crackdown might be good for you. Particularly since it's pretty old and I'm sure you can find it for cheap. You level up enough and you can throw cars and leap from building to building as you rain rockets down on the street.
Bioshock was good, but a little disappointing for me. A great start but ultimately doesn't really deliver. A good rpg and interesting underwater city to explore.
Halo is Halo. A fun cinematic fps with entertaining enemies to kill. The AI may not be brilliant, but they're smart enough to be satisfying to kill on the harder levels.
If you have absolutely any interest in Boxing, then fightnight is a great game to pick up. It's out there for cheap.
I hear the Rainbow Six:Vegas games are good tactical shooters if you're interested in that. If you're more interested in squad based tactical shooters then there's the ghost recon games.
