The Core i7-875K seems to be more than happy to run at high clock speeds on the Maximus III Extreme motherboard we used for our testing. We found that we could do quite a bit even at 4.5GHz.
However, the problem came when we tried to test anything that pushed all cores/threads. When this happened things broke down very quickly. We did try to pump a little more voltage into the system, but as we were only using air cooling, we did not want to push too far. In the end we set the CPU and voltages as you see below.
Again, we have a feeling that with better cooling we would be more comfortable with pushing the voltage up and getting much higher clocks.