RV770 is the chip behind ATI's Radeon HD 4870 card and it showed quite good performance, so this 1.6 times performance increase can give you a general idea that the new chip should be quite fast.
Of course 1.6 times is the best case scenario and it should be less than that in some cases, but overall this 40nm chip should be a good step forward for ATI.
The big issue is that by late November Nvidia might launch its Geforce GT300 chip, that might end up even faster, but at the same time Nvidia's DirectX 11, 40nm chip will also be bigger and probably hotter as well.