Depends on the specific situation, not all CPUs are created equal. Some need higher voltages than others. Even if they are same make and model, so that question can not be answered exactly. Like my Duron 1300, I have it OCed to (as I check SiSoft because I forgot what I OCed it to lol) 1417 MHz, and I undervolted it from 1.75 Volts to 1.69 volts. I wish the 1300 could be unlocked, I know this thing will do 1500, at least, maybe as high as 1600.
Water cooling is for the weak. Get liquid nitrogen.