Cayman based 69xx will be ~45%(1.2 for the IPC x 1.2 for the SP count increase) faster than 5870.This alone is enough for it to crush any GF100 based single GPU card.
OOOOOOOMG you have this card HD 6970
you say 45% ...... ok GTX 480 faster than 5870 around 26%
45%-26%=19%. that Mean Cayman XT (6970) faster than GTX 480 Only around 19%. so if the GTX 580 (look here boy) come faster than GTX 480 around 20-25% ........ that Mean GTX 580 will be faster than Cayman XT (6970) around 6%

Of course, all this talking expectations