I have a board that allows cores to be locked or unlocked, so I would say it's motherboard-dependant. Of course, when I tried to unlock a core on my fx 4100 chip, the computer wouldn't boot, but my guess is that there just isn't a 5th core on that.
Anyway, yes, you should be able to disable them if you have the right board, and it shouldn't damage your processor. Also, can you not pump that baby to 4.3 ghz on stock cooling already?
You guys completely misunderstood what the OP said.
You can't take a 4000 series or 6000 series chip and unlock cores that AMD locked at the foundry. BUT you can lock cores that are active at purchase.
For example, you can't take a FX 4100 and unlock the disabled 4 cores/2 modules, but you can take a 8150 and shut down 4 cores/2 modules. Thats what the OP is talking about.
As for the OP's question, it wont necessarily give you higher overclocks. It will give you lower power usage and it MAY give you higher overclocks. But its not automatic.
Keep in mind you can also disable complet modules and run 4 core/2 module or disable the second core in the module and run 4 core/4 module. 4 and 2 gives you less power usage, 4 and 4 gives you better IPC performance.