If you are really dead-set on another card, check the buy/sell/trade section as I'm sure you can pick one up for a better price than buying new. But as others have suggested, adding a second 560Ti won't give you much of an improvement. Your screen doesn't have a high enough resolution to really get a decent FPS improvement from adding a second card.
edit: someone beat me to it...
Might I suggest you try sell the 560Ti, and use the proceeds to get a 660Ti. (I'm not much of a fan of multi card solutions.
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