I have a Phenom II X2 560 Black Edition CPU from AMD and I want to know how to unlock it to 4 Cores completely stable, I'm using the ASUS M5A88-M Motherboard and I would really appreciate an answer with specific voltages and all that, I'm also using Sapphire HD 6790 Graphics Card and for RAM I'm using Corsair Vengeance 8GB (2 x 4GB) and the system is powered by Cooler Master eXtreme Power Plus 500w PSU.
For unlocking, you do not really need to adjust any voltage as long as you stay at stock. However, it is no guarantee that you wont have too because the characteristics of processors vary chip by chip.
For your motherboard, "the Core Unlocker feature simplifies the activation of a latent AMD® CPU—with just a simply switch. Enjoy an instant performance boost by simply unlocking the extra cores, without performing complicated BIOS changes." so it should be easy for you. Good luck.
As RK mentions the only way to tell if you can get a stable unlock is to try it, test it and figure out the best option your CPU allows.
It's all down to luck if your CPU will unlock and run stable.
To get four cores up and stable in overclock you basically have to get lucky 3 times. Once to unlock each core and have it be stable, and once more to be able to overclock and have them remain stable.
Before you unlock all 4 cores, you need to make sure that if you need any extra cooling that the stock fan or other coolers can handle the heat. Also, surely if the other cores were fine, why would they disable two and not sell it as a quad core? I think that the extra two cores may not be as potent as the active two or they were too lazy to build real dual-cores like they did with their latest line (FX) using integer cores
Also, this article from AnandTech shows how to unlock the AMD Phenom II X2 555 which may be of some use: