OCIA.net has just posted an interesting new article titled: "Build A Top-Notch Gaming PC Designed for Overclocking for $700". Here's a taster:

For overclocking enthusiasts, this CPU offers tons of fun for less than $100. Starting at 3.1GHz with a 200MHz frequency and a 15.5x multiplier, it's a Black Edition CPU, meaning that you can overclock it simply by raising its multiplier.

With the ACC Hybrid Mode, many were able to unlock two cores and get a very inexpensive quad-core CPU. However, keep in mind that not all X2/X3 CPU cores will unlock, as some are truly defective.
Also, with nothing short of four great AMD CPUs within $9 of each other, here are three other options that you should also consider:

 1. AMD Phenom II X2 555 Black Edition 3.2GHz Dual-Core C3 Stepping 80W - This particular CPU brings in slightly better thermal management with the C3 stepping, compared to the X2 550, allowing for higher maximum frequencies when overclocked. Also, good to know, there were reported successes of unlocked cores, allowing for a very cheap quad-core with an unlocked multiplier. With the highest frequencies, dual-core processors tends to currently be the best for video games whose performance highly depend on clock speeds.
 2. AMD Phenom II X3 720 Black Edition 2.8GHz Triple-Core 95W OEM - OEM means that it doesn't come with a stock cooler (You'll be using the Cooler Master Hyper 212 Plus, so this not a problem) and that it comes with a 30 days warranty instead of 3 years: Not a problem again since overclocking voids the warranty ;) More importantly, for only a few dollars more, you get an additional guaranteed core while keeping the benefit of an unlocked multiplier (Black Edition CPU). Like the X2, this CPU can have its 4th core unlocked to become a quad-core CPU, although once-again, this is not guaranteed. With 3 cores, medium frequency and an unlocked multiplier, this is a well-balanced CPU that is good for gaming as well as CPU-intensive workloads such as video editing or 3D.
3. AMD Athlon II X4 630 2.8GHz Quad-Core AM3 95W - Only a few dollars more than the Athlon II X4 620, the 630 brings in the benefit of a higher multiplier (14X instead of 13X), an important thing here considering that the multiplier is locked with this CPU. Indeed, this is not a Black Edition CPU like the other suggestions, however if you want a guaranteed quad-core CPU, this is the way to go. Also, some consider the lack of L3 cache a blessing for overclocking, because an Athlon II produces less heat than a comparable Phenom II processor, meaning that your temperatures will be lower and less likely to limit your overclocking. Since this is a quad-core CPU, it is capable of handling gaming without a problem, handles CPU-intensive workloads better than dual or triple-core CPUs and is considered by most to be the most future-proof solution.