what size PS are you using? I use the EVO Galaxy 1.2kW. This ensures I'm good for at least a couple of years before I need to upgrade the PS

One thing to check is that your PS rails can supply the needed amperage to the video card. Some PSs can't push the needed amperage from some of the rails in it - you have to use specific rails for video cards.
Another thing to look at is mobo settings. Some mobos let you limit the amount of power the video card can use. Some Asus mobos are this way.