The GTX 580 is nvidia's fastest single-gpu at the moment. The 570 is about 15% below it. These are at stock clocks mind you.
Right now AMD just released their 79xx series which are new generation graphics cards. The flagship 7970 outperforms the gtx 580 by upwards of 25%. However, new generation Nvidia Kepler graphics cards are on the way and on paper, will outperform amd's new 79xx series by considerable margin. The best thing to do unless you need a graphics card literally RIGHT now, is to wait until more is known about Kepler.