CPUs do not use crystal oscillators internally to generate their clock (although there is no technical reason that they cannot). Typically the clock source comes from a pulse generator of some fixed low frequency (sometimes a crystal but it can be a purely electronic circuit created using analog components) and this low frequency pulse train is pumped into what's known as a PLL or Phase-Locked Loop. The PLL is responsible for multiplying the input signal into a number of configurable output signals. The output signals are a function of the input signal's period/frequency, a scalar multiple and a phase angle. All parameters need to be known or found by comparison in order to avoid nasty causality problems.
If a PLL is configured to create two output signals with a frequency 10 times that of a 50Mhz input signal, with one signal Pi radians (180 degrees) out of phase with respect to the source signal then the PLL will create two 500Mhz output signals, with one trailing the other by half a period.
The reason some CPUs overclock better than others has purely to do with silicon quality. Not all CPUs are manufactured equally, however CPUs within a family are manufactured identically. The best manufactured ones are taken and packaged as the top end CPUs and downward from there. CPUs that have physical defects which would cause them to malfunction when operated at full capacity may have part of their functionality cut and then be sold as a lesser model. The famous Sandybridge-E 3960x and 3930k are actually both fabricated as 8 core processors with 20MB of L3 cache. Both processors have 2 cores disabled for yield reasons (the 8 core Xeon models cost thousands of dollars) and between 5 and 8 MB of L3 cache disabled.
What sometimes happens is that silicon yields and market demand diverge. Either good silicon yields are low and top end products cannot be manufactured in sufficient quantities (this is what's holding back the GTX 680 right now, yet is resulting in tons of GTX 670s) or silicon yields are high and far more top end products can be manufactured than necessary (this tends to happen as a manufacturing process becomes more mature). In the latter case the manufacturers take perfectly good high quality chips and package them as weaker processors to meet market demand. This is why some AMD X2,X3, and X4 processors can be unlocked to X4 and X6 processors yet others cannot. Some were binned down due to the presence of defects, others were binned down for marketing reasons.
So that explains why some chips overclock better than others, but what about the effects of overclocking?
All electrical circuits undergo a process called electromigration. Electromigration occurs when current flow cause by an electric field physically damages the conductive path that the current flows through. This doesn't only happen in CPUs, it happens in electrical wires everywhere. At low temperatures the crystal structure of the conductive paths in CPUs is strong enough that high current wont dislodge the atoms from the structure. At higher temperatures, or at higher field strengths, the flow of current places a much greater force on the conductive path and degrades it quicker. Eventually this can degrade the conductive paths to the point where the CMOS logic network cannot switch states fast enough and the only way to rectify this is to feed it more voltage which in turn furthers electromigration. Eventually, the conductive path will break completely and render the circuit useless (burned out). Electromigration happens whether or not you are overclocking; overclocking will simply worsen it exponentially. So yes, overclocking increases the rate at which the silicon sustains damage.
I hope this helped
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