It is safe to assume that a power supply (PSU) of 400 - 600 watts has ample power for your PC. Without a video card that draws power 400 watts is plenty. But with onboard PCI Express x16 2.0 drawing power and requiring four and six pin plus 12 volts, the game has become more interesting. Now with Active and Passive Power Factor Correction (PFC). Your apple store geek will tell you this saves energy and money. Hopefully at least it will put less wasted heat into the room, saving on air conditioning and lessening degradation to plastics and non metallic components in the path of said heat. Power factor correction (PFC) helps smooth the current draw along the sine wave of complex AC loads. Read about the PC PSU switchmode power supplies. Simple loads, like the electric oven, draw a constant current along the time axis. The intent of an active or passive PFC is to make them act more like simple loads. In a graph of voltage versus current drawn for the electric oven in an AC circuit, the current and voltage are in perfect sync. For a complex load, this is not the case. For a complex AC load the current flow is not in sync with the voltage amplitude. The more complex a load is, the more out of sync the current can be with the voltage, and the worse the device's "power factor" will be. An active PFC PSU will waste some power (at least 10%) as heat, just like every other circuit in the world. A non PFC PSU will make no attempt to synchronize the current flow with the voltage amplitude along the time axis and thus not waste power as heat to do so. On the other hand, it is possible that some active PFC PSUs may deal with lousy waveforms better than passive PFC or non PFC units, but PC PSUs generally handle spikes and surges and dropouts pretty well.