Well here is a 570 vs 7870 and you can see first hand how they compare. I thought the question was how many amps does it need to power up? Well I could be wrong but here is the link I hope this helps.
In this link does show how much better the 7870 is and it is better than the 570. Again the new nVidia Kepler will be out in a few weeks. So the 670 might give the 7870 a run for it's money.