i am no electonics engineer but here is my understanding on that crystal thingie.

That is the base clock unit that determines ALL the frequency function in your electronics. When you overclock or underclock, it is based on this "crystal". Without this crystal, the whole board will not have a sense of timing..like 1hz or 10hz and so on. So when you modify your GPU frequency, it is based upon this crystal. And no..you cannot modify the 24.576Mhz timing in this clock. It is dead locked.

Clear?