1. Yes with an ACC enabled board.
2. Cores were defective due to manufacturing error
3. With making anything some F*** ups are worse than others and hinder functionality like a motor it might run but not as intended or not at all. The bad cores are made not to be used. ACC is a trick to get around the disablization. Some good 4 core cpu's just end up disabled to meet demand for 2 and 3 core cpus while all 4 cores that had bad cores get disabled and sold as 2 and 3 core cpus. This allows product not to go to waste.
4. If a core is unlocked and proves stable in the beginning most likely it will remain not to say its not impossible to become corrupt even good 4 cores sold as 4 cores can go bad though I havent experienced it.
5. Athlon 620' are hitting 3.8ghz and more if you are comparing them to a black editionX3 it will be more envolved to reach these speeds with Athlon.
6. I personally bought a 620 for $10~$20 less than a 630 overclockers either make cheap parts work like better ones or buy the best to make the absolute best its personal choice. The higher multiplier of the 630 which is 14 makes it more appealing if you have concerns or know that your memory or motherboard does not work at high FSB speeds. The stock FSB is 200mhz on AMD's and then is multipied 200x14=2800mhz for 630's.
7. Yes and No again the reason the 4 core was demend defective may have been due to just one core not meeting AMDs tolerances thus gets turned into a 3. With AMD through my experience you almost always can bet on overclocking atleast 25% with even basic motherboards and memory. Through observation of others with X2's/X3's they usually seem to be able to clock higher on the orginal number of cores. This may due to heat issues because when unlocking cores one usually has to increase the voltage for them to become stable but this usually puts the chip at the same voltage as its 4 core counterpart. Then yet again voltages increase with the high clock speeds as well but it would seem that it would be no different than its 4 core brother. Basically you take a X3 720 BE unlock it and its stable you will get to overclock it about as high as a X4 925 since both are 2.8ghz stock and the 720 most likely is a 925 turned 720.
More Advanced Micro Devices than Intel