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.