sky, it sounds like you're not into multiplayer much. In which case, you'll probably want to avoid COD4. Even thought it does have achievements and such for replayability, you seem like someone who only likes a game if it's 1) extremely difficult and 2) takes a month or two to complete. Most people don't like games that take that long since they've moved on to the next great game in that time.

For the average gamer that doesn't spend 40 hours a week playing, COD4 is excellent. It's a bit short, but the multiplayer aspect more than makes up for it.

As for me, I just picked up Left 4 Dead (on sale on Steam, 25% off). I can't wait to play that. I'll probably also pick up Bioshock since it's only $5 on Steam (regularly $20). I'm thinking of getting COD WaW, but it's only $5 off, so I'll probably hold off on that.