That is true only if the power supply is restricting the performance of the card. For example, if you have a low wattage power supply running a high ended graphics card, then the card will run in a reduced mode. For example if you have an 8800GTX with a 550W power supply and then upgrade that to a 600W one, the performance would not change. The PSU doesn't really have any bearing on performance, its only when there isn't enough juice that performance is affected.