It depends, unfortunately. If the Q9400 has an unlocked CPU multiplier, you could OC it simply by raising the multiplier (Base clock x multiplier = CPU clock). If not, you have to raise the base clock itself, which will affect other components as well. All of this is dependant on your CPU model and what the motherboard BIOS will allow you to do. It also helps to have decent cooling so your CPU doesn't overheat from the (potentially) increased power output.