To answer your question: Black editions have unlocked multipliers, making it easier to overclock.
Here's my 2 cents advise: If I were you, I'd rather pay 80$ and get a brand new quad-core CPU rather than pay 45$ and try to unlock the hidden cores on a used dual-core CPU. I heard it doesn't work every time and some people reported stability issues after waking up the "hidden" cores. But I don't know what's your budget, if you are tight you might be willing to take the risk (I'm not sure how big is the risk though, some other people might step in and give their thoughts, looks like a lot of people were able to achieve that successfully too).
Also, the question by inerax is still valid: What are you planning to do with this computer? Is it for gaming or just browsing the web? Because if you don't do much with the PC you're not likely to see a big improvement going from 2 to 4 cores. However, in some games it can make a huge difference (most modern games really like the quad-cores).
Finally, if you change your CPU, make sure your current BIOS version supports the new CPU. If you don't want to take any chance, just flash your BIOS before swapping the CPUs.