I have a Gigabyte GTX 560 Ti SOC Ed. and it comes with a factory overclock of 950Mhz on the core. Though I have yet to need doing so, apparently they it can handle ~1GHz (core clock). At standard fan rates (40-60%) you can barely hear them, at maximum fan speed, there is still very little audible noise; they're quieter than my case fans. This is on account on the custom cooler Gigabyte use, very similar to the Twin Frozr II, so it should be easily sufficient for a little overclocking.
Good luck OC'ing