well I can explain the unlocking part,
When AMDs are shipped out the mulitiplier in the BIOS is fixed to whatever say for arguments sake 12.5.
The fsb is also set say 133
Now if you do your math, you know what 12.5x133=
well you work it out.
Now that is the cpu speed.
When people refer to unlock they tend to mean that they wish to unlock the L1 bridges, so that when they go into BIOS, they may increase and decrease the multiplier, resulting in a higher then what you got it at speed.
One other thing to consider is that when you get one, look at the stepping to make sure that it is one that you may overclock, as there is nothing like getting one and finding out that it is the worst one of the bunch.
Search theses forums, to find out which is the best stepping in it class, and all your other needs..
Almost forgot, if you do overclock..REMEBER, make sure your cooling of the cpu is up to scratch as when you overclock it shall give out more heat.
Hope this helps