Exactly what was your old processor?
If it was running at 2.1ghz like you say, the 3800X2 should be better, but it is not going to make a huge difference over what you had.
COD is not going to use the dual cores, but the 2 cores will handle background tasks better, leaving more CPU power for the game.
Are you sure your processor is be detected by the BIOS and set to the correct speed. You have 2 processors showing in your device manager, and 2 graphs in your task manager under CPU usage?
Disable cool and quiet. In my experience cool and quiet does have a negative impact on gaming perfromance. It shouldn't, but fact is it does.
You should not need the dual core optimizer installed for modern games, but go ahead and install it just to be sure that the timestamp issue some older games have with dual cores is not the problem.