FUD Zilla published some details about the improvements ATI made in the upcoming RV670 core:
It was a time for a new strategy. The RV670 is a redesigned and improved R600 core done in 65 nanometre with official support for PCIe 2.0. The base R600 design is improved with UVD and this is a something that was missing from this big card.
ATI added Shader model 4.1 and DirectX 10.1 support for this new core. At the same time the company claims that it managed to improve antialiasing performance and at the same time that it managed to reduce the memory interface bandwidth. The anisotropic texture filtering quality is also improved and at the same time ATI managed to reduce the transistor count.