I'd try to find another 2gb stick, just make sure it matches the sticks that are already installed. Spending that much for 2GB more is kinda silly unless you're running out and can't find another 2GB stick.
Assuming you've got it set up in 3 channel right now, I'd say just get 2gb of 1333. You're not going to see much of a performance difference between 1333 and 1600, especially for gaming.
You will want to try to figure out the timing and voltage of your current ram, or just get the exact same model.