Now ATI graphics cards are back in favor it seems AMD will slow down GPU development a bit. Nordic Hardware claims you shouldn't expect anything radical from the upcoming RV870, this graphics card will arrive in Q1 2009 or later and while it will be faster than the current Radeon HD 4800 series you shouldn't expect a huge jump in performance.
The only real specifications that have been going around is the very loose rumor of 2000 shaders, and it doesn't look very likely. AMD is looking to master 45nm with its GPUs first of all and further improve its multi-GPU design, before doing anything radical. The RV870 will be faster than RV770, but don't expect the same jump we saw when moving from RV670 to RV770.