They don't consume 28A. That's 336W which is even more then my 7950 uses and more then the power plugs can supply. For some models anyways. 28A for the entire system maybe, but even that seems high. They are rated for less then 200W, so anything that can output 550W on the 12V rail will do. Double check that you have enough PCIe plugs for that setup with the cards you want to use.