You may have never found the opportunity to bother about the power consumption of your computer in the past. However, this is one point that you will have to consider after you overclock your processor or your radio card. Why? Well, over clocking is going to generate extra heat and you will have to install extra cooling equipment to manage the problem.
In such a scenario, the extra fan or water pump, if you use a water based cooling system, will function through these same SMPS that powers your computer. If you notice, your computer fan is powered by your motherboard, which in turn derives power from the SMPS, which in turn derives power from the external power source.
You will either have to go in for an independent power source that connects the cooling fan directly to the external power supply or you will have to ramp up capacity of your motherboard.
In both cases, you will have to consider the extra power consumption. Your processor may function properly despite overclocking but your motherboard may just not be in a position to manage the extra voltage requirement. Use the web to identify whether the motherboard is capable of handling the extra load or not.
Contact others who have done this in the past and find out the various precautions that they have taken. In some cases, people adopt a conservative approach and end up increasing voltage requirement even though the processor does not require extra cooling to such levels.