Taiwanese overclocker Luca has published an overclock with AMD’s Phenom II X6 T1090 Black Edition CPU where he pushed the processor from the stock clock of 3.2GHz to 6.29GHz. The cooling was of course liquid nitrogen and the motherboard was ASRock 890GX Extreme3.
Judging from the verification screen he needed 1.928V to get there, which was a big increase from the stock voltage, 1.32V. Even you should take this particular number with a pinch of salt since it’s just registered through software.
Interestingly many overclockers have achieved over 4GHz with just air cooling. The entry level Phenom II X6 T1055 was moved to 4.4GHz with just air, which isn’t shabby at all considering it is expected to cost around $200.