[Ludacrisvp]

varying quality with the manufacturing of your phone will determine how much you can overclock your device. it doesnt mean that you have a bad phone if it cant do 768mhz since it does 528mhz (what it was designed for) just fine.
its possible that your phone could run 767 or 792 or some other random speed that clocks it just right and doesnt crash. example is i can overclock my mac pro 2.8ghz 8 core to 3124mhz and it works fine but it locks up at 3100 thru 3123mhz..... you just have to find what works for your hardware the best.
one other thing mentioned by phhussion is that some peoples overclocking is determined partially based on power source. on ac adapter your device may be able to do 768 or 800, but it may crash when using battery only.