According to theoretical numbers RV670 will be as fast as R600. I guess this is ATI's dark secret as RV670 is indeed an improved version of its R600.
The RV670 at 750 MHz has 480 Gigaflops processing rate, while the R600 at its 742MHz has 474Gigaflops which implies that the chips will have the same performance at the same speed.
ATI's dark secret is that at maximal 825MHz the chip can gain an additional 50+ Gigaflops but we still believe that the RV670 has to lose to R600 in high resolution with FSAA and Aniso due to its 256Mbit memory interface.
RV670 in its three iterations comes in November.