I would to like to end all the ambiguity of how the GTX 660 OEM performs. With my rough benchmarking estimates, you are getting the performance of about the HD 7850, sometimes better and worse depending on what programs, games and settings you are using. Overclocking potential is also limited, and the cooling is quite bad when overclocking.
I bought a Dell XPS 8500 with an i5-3450 and GTX 660 OEM for $700 to replace my 10 year old desktop with the intent of playing some current games at high settings. I figured the 660 OEM would be a powerful choice for the graphics card. Then I ran some benchmarks and I realized how much lowering the core and memory clocks hurt performance. My 8 year old monitor has a maximum resolution of 1280x1024, so I am forced to work with 1280x720 with the exception of 3DMark11 Extreme.
Unigine Heaven 3.0 - 1280x720, Default Settings
Min FPS: 11.9
Max FPS: 221.4
Sleeping Dogs Benchmark - 1280x720, 75 Hz, Extreme Settings
Average FPS: 53.6
Minimum FPS: 29.9
Maximum FPS: 83.2
I also did a mild overclock on the card. Using MSI Afterburner I added 80 to the core clock and 300 to the memory clock. The temperature shot up to 95 Celsius at one point. The power limit can be pushed to 140%, but why bother when the card will stutter before drawing so much power anyway.