It's going to heavily depend on the card. Each card and brand are going to respond differently to overclocks as well as temps. There's a 570 overclocking thread here
with most people using the EVGA Superclocked card. But as others have said, overclock to what your card can handle, and with temps that you're comfortable with.
My 570 overclocks to 975 core and 2300 memory at 1.075 volts, but runs up to 74c which isn't a temp I'm comfortable with. Some might say that it's silly, but when the card runs 850 core 1900 memory at 0.95v at 59c perfectly fine, I don't need to run such an insane overclock all the time.