ASUS's GPU Tweak overclocking utility does a fine job of tweaking and monitoring parameters, with facilities for saving profiles, screen/video capture and update notification for new bios and driver releases. 

Due to Nvidia's controversial Green Light Program, AIBs are forbidden (if they want to provide a warranty) to manufacture Kepler based cards with any capability to raise voltages above the official 1.175V limit, thus rendering any meaningful overclocking moot. Furthermore, the TDP power cap is limited to only +10% (127W), which greatly neuters the GPU boost clock frequency headroom.

  Reference GTX 660 ASUS GTX 660 DirectCU II TOP ASUS GTX 660 DirectCU II TOP with OC
Base Clock 980 MHz 1072 MHz (+9.4%) 1137 MHz (+6%)
Boost Clock 1033 MHz 1137 MHz (+10%) 1202 MHz (+5.7%)
Memory Clock 1502 MHz 1527 MHz (+1.6%) 1677 MHz (+9.8%)


Overall, the cooler was effective and quiet, which is not a surprise given the relatively small heat output of the GTX 660.