I'm going to be getting the AMD Phenom II X3 720 for the computer I'm building and I've heard something about being able to unlock a 4 core.
Is this just a form of overcloaking? Or are you really able to unlock anther core? And if so how do you unlock the core?
Asrock makes a board that unlocks cores, but there's no guarantee that the 4th core will function properly. It may also affect overclocking. Look for a 750 or 710 southbridge chipset and read some newegg reviews.
All of the Phenom II processors are made as 4 core processors. Then the fabrication plant tests the processors. If a processor has four good cores it goes one way, if it has three good cores it goes into a second batch. I'm not sure about the two core processors, they may be made as two core processors, or they may be four core processors with two bad cores. When AMD first started producing Phenom II processors, the yield was low, and a lot of them had bad cores. As they continued, the yield improved and fewer processors had bad cores. They started taking processors with four good cores and turning off a core and selling them as three core processors.
So what is a bad core? It can be a core that won't function at all, or it can be a core that won't function reliably at whatever over-clock AMD uses to test it's cores.
If you get a chip with four good cores, then you can turn on the core that has been turned off, and you have a real prize. If the core did not function reliably at the test over-clock, it may (or may not) function reliably at the stock clock. If it's just a bad core and won't function at any speed, oh well. They sold it to you as a three core processor, if you can't get the fourth core to function you have no complaint.
Although I did read a review on newegg by some moron who had taken away stars because he could not get the turned off core to function.
If I had an AMD processor with a locked core, and a motherboard that could unlock it, I would certainly try, and you should too. If you can unlock it, consider it to be a bonus, but if you really need four cores, buy a four core processor.