What makes a CPU a good overclocker?

[SideWinder]

Cache? FSB? etc. Thoughts.

 

[SideWinder]

Luck.

And the ability of the overclocker.

But what makes a CPU a good overclocker, how and why does it have that ability?

For instance, why do the E2xxx CPUs clock better than the E6xxx ones? Because they have a lower stock clock?