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.
the best value atm is the gtx 570 they can be had for less than £250 or $350ish your gonna pay 40-50% more for about 25 percent performance increase on the 7970 the nvidia cards will likely launch at around $450 for the 670/770(depending on how they label it) the 680/780 is likely to come it at $550-$650+ or £500+ going by current pricing to performance.
they may surprise us by pricing them competitively to replace the current 570 and 580 but thats unlikely as there yields are supposedly to be pretty low at launch.
The 3 top GPU's for Nvidia the 570, 580, and 590 even though it is a two core card. That is if your budget calls for those cards. The GTX 560 Ti's and the GTX 560 Ti 448 cores are at the bottom.
The average card with the average price is the 560 Ti's 1gb card. The 448 core one is becoming obsolete, they are faster than the 560 ti and in some cases were they are OC'd faster than the 570.
The new line of Nvidia cards are do to hit the shelves in the next two months. They are the Kepler 6 series. So if you are not in a rush then wait and see how these turn out. Anyway I wish you good luck on your decision.