GPUs haven't been advancing by huge leaps and bounds lately. The difference between a top end card this generation and a top end card last generation is around 30%. So you do get overlap where a performance tier is simply moved down on the existing hierarchy eg. 6850 is now about = to 7770 and 7850 is now a bit better than the 6950.
For AMD cards pay more attention to the second digit to get an idea where the performance is at. Same with Nvidia. The higher the second digit is , the better the card will perform.