GTX480 ~ GTX570. GTX580 is faster then the GTX480.
As for power I'm not sure. I think the GTX560TI 448 might use a bit less so you might be better off with that card. Overall they are probably close enough in power as well that its best to just get whatever is cheapest.
They are both around 225W. GTX480 I think is 250, not sure on the 448. 500W would be needed just for the GPUs, more if you are OCing. Add in your normal 100W quad CPU and you are at 600W+ for an SLI setup. A good quality 750-850 should be able to do it.