I you're really looking for games on a budget, you shouldn't buy recently released games, unless you plan to play a lot of multiplayer. You could take a look at some of the better games released last year, and look for a good deal. Of course if you have to buy something new, I would guess AC2 if want a singleplayer experience, or otherwise either MW2 or L4D2, depending on what kind of multiplayer experience you would like.