I've only bought 1 game this year due to Gamerang rentals, Halo 3. There are 3 upcoming games, however, that have potential for high multiplayer replay value.
Civilization: Revolution - Civ on Xbox live? With reportedly intuitive controls? Where do I sign up? Any why haven't you guys been barking about this one?
Lego Indiana Jones - I'm having visions of competitive Pitfall here. With blowdarts!
Culdcept Saga - CG/Boardgame with a magic feel. minus the collectible aspect. Sounds like a winner to me.