Wattage is a bit misleading. Without getting too overly complicated, basically each Voltage on a power supply has a maximum current on each line. Each voltage maxxed at each amperage attains a wattage (Volts x Amps = Watts). If you get a power supply that is double (or more) of what is suggested you generally never run in to these problems and efficiency might also be better. If you have a power supply that is also rated as PFC, even better. Look at it like this: If your car has an engine that red-lines at 5000 RPM, is that where you want to run it all the time, or would you rather run it at 2500-3000RPM? Is a car more fuel efficient at 80 MPH or 55 MPH? As speed increases, so does win coefficients. As Current increases, so does heat and strain on the components themselves. I never run anything less than 800Watts PSU on my rig, even though it is probably rated at 450 maximum.