Okay, if you want a game that is a cross between those two games then look no further then Fallout 3. It is an RPG and a first person shooter. It is an excellent game and a nice choice. This has no xbox live and no co-op. However, it's as long as Fable 2 so that doesn't matter. This is a great game.
If you want a great shooter game, maybe the best ever made, then get Gear of War 2. You do not need to play the first one to like it, but you will like it more if you play the first one. It is a simply amazing game, and a MUST HAVE, for any gamer. Not to mention it has the best xbox live ever!!!!! The storymode is also great!
Now anyother great multiplatform shooter is Call of Duty World at War. It's got a good campain and a GREAT multiplayer. It is a great option for any shooter fan. The only thing I would advise is even if you dont like World War II games (this is one) you still might like it. It takes World War II to a whole new level.
Next up is a horror game. Left 4 Dead is not incredibly long, but makes up for it by being flawless. It is scary, and rediculusly fun! This is being called the best game of 08 and I'm not arguing! It's a good game, but is one of two games on the list that has no team slayer live, but you can play co-op both split screen and on live.
These are all really good games, but the best is Gear of War 2! Talk about gore, this is IT. It is sooo bloody and good. You can chainsaw people in half! Anyway they are all good games.
As for your addiction... join the club (lol).