It might be faster than your SLI setup but not by much.
A single GTX 660 has roughly the same logistical demands as a single GTX 550 Ti. I would upgrade if you're unsatisfied with variables like power consumption and temperature, or if your SLI setup is unstable for certain games. I wouldn't upgrade for the raw performance though.
Here's another way to think about your situation. The GTX 480, GTX 570, and GTX 660 have very similar performance (480 vs 570[anandtech.com], 570 vs 660[anandtech.com]) and the frame rate delta isn't very high. The actual gains are in other areas.
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