The current 900Mhz 560 Ti @ $210 is currently slightly faster than the $260 7850 ..... the 7850 finishes right between the "reference 560 Ti" and the "non reference 900 Mhz 560 Ti" models. The 560's were step above the 68xx series and competed well with the 6950.
I'd venture to guess the that 660 Ti will be at the 7950's performance and the 650/660 will sit on either side of the 7850. But, with stuff that's not here yet, this is all conjecture at this point. Until we see the numbers, that' sall anyone can do.