Quick rundown

Any Intel chip since about the time of the P2 300 (except engineering samples) is multiplier locked, and you can't unlock it. Your only option when overclocking an Intel chip is to boost the bus speed. Make sure you have high quality RAM and peripherals (especially hard drives - Maxtor drives are notorious for being poor overclockers). Increasing bus speed will obviously put more stress on the RAM, but PCI devices will also feel some strain, since the PCI speed is determined as a certain fraction of the bus speed. When the system starts to get unstable at higher speeds, it might help to increase voltage. Go by the smallest increments you can, and always keep an eye on your processor temperature. You DEFINITELY need a high-quality heatsink/fan combo for overclocking. I don't like to see a chip over 50 deg C.

It is possible to unlock the multiplier of AMD chips, which is one reason they are more popular around here. Then you can overclock either by increasing bus or changing multiplier (or both).