When running the processor at much higher speeds, the processor core voltage needs to be increased. Stable System is What You Need Start in small increments. Bump up the bus frequency or multiplier, and see if the system boots. If it boots, run some processor intensive applications such as Quake for an hour or two. Don't overclock a system that crashes every hour.
Stability is the key word.
When hitting the upper peak, and the system can not boot up, check the temperature of the heat sink. If the heat sink is really hot, then a better cooling system is needed to run at that speed. Higher-performance graphic card definitely helps.