According the Top GPU page.

http://setiathome.berkeley.edu/gpu_list.php
http://gpuboss.com/gpus/GeForce-GTX-660-vs-GeForce-GTX-570

The GeForce 570 has a better average time to complete tasks.
The GeForce 660 is a newer card with better specs.
Can someone explain how the older 570 can do better than a 660 on SETI?
CPU Clock? 3D rendering?

I wouldn't take the results on that page as being very reliable. Still, it's worth nothing that the GTX 570 uses the high-performance (of its generation) GF110 GPU while the GTX 660 is a mainstream GK106. Even though Kepler generation is much more energy efficient and generally better performing in the graphics domain, for GPGPU I found Fermi to be better than most Kepler GPUs (albeit at the cost of greater power consumption). For example my GTX 580 generally runs better in GPGPU than a GTX 670 that I used to have, even though the GTX 670 would normally do substantially better at graphics processing.

---

Soli Deo Gloria