Every chip varies a bit so some can overclock higher or with less voltage than others. I personally have stuck with 4.45ghz @1.43v (1.41v under full load) because i didnt want to push the voltage close to 1.5v and wear things out faster. Past this point for me it starts taking alot of voltage for just a small clock increase. Ive seen some people that say they could get close 5.0ghz with the same voltage, so its all in luck of the draw.
I also had an issue with my old board's vrms not being good enough and i had to upgrade from my old asrock extreme3 to my current gigabyte 970a-ud3p. Previously i couldnt get past 3.8 ghz on 8 cores without the vrms throttling down from getting too hot. These chips use alot of power and that power first goes through the VRMs. So your board can be a bottleneck before the cpu would be if its not built strong enough for such increase in power. Temps is also something that will likely be a limiting factor. WIth my corsair h80 and push/pull fans i wouldnt have been able to reach my current clock. With my h105 with push/pull and high static pressure fans my temps never go above 45*C. Most say to keep full load temps under 60*, ive heard 70*C for the socket temps (not core temp) are fine but thats pushing things. I also have a TX750 power supply and im fine