If you want to play online multiplayer, I would say Call of Duty 4 is where it is at. Although I haven't played it, I've heard good things about it and it's available on Mac, PC, and console. Modern Warfare 2, one of it's offshoots is coming out soon on console.
As for Quake 4 vs Quake Wars, I haven't played Quake Wars, so I can't respond about it. Quake 4 was okay, but I played better. Unreal Tournament 2004 has better online play. The single player was good though.