We did not have a card for our own testing at launch, so we will use NVIDIA's numbers for now.
Shadermark renders a low polygon count scene using different shaders. The speedup here is between x1.5 and x3 compared to a G71-based card.
NVIDIA tested a large number of games and benchmarks as well. Very nice numbers here. Compared to the 7900 GTX the average speedup is about x2.0.
Against the 7950 GX2 the differences are smaller. But you can see huge performance improvements when HDR rendering is in use.