We learnt that ATI's Catalyst team is now working hard to improve the performance of RV770 drivers.
With current driver ATI can get, there's no obvious performance differentia between 4850 and 4870 with same clocks. So this means the current driver can not dig the potential of GDD5 out of 4870. Graphics Cards maker in Taiwan told us that the GDDR5 video ram sample of RV770 came from Qimonda with 512Mb(16Mbx32-bit) configuration and 4GHz default speed, but due to the complexity design of the 4870 PCB which is not the same as 4850 PCB, and the immature driver, now 4870 can not reach to 4GHz.
If the video ram of 4870 can not be steady at 4GHz in time, ATI may use the alternate plan to use GDDR5 video ram from Qimonda at 4.5GHz default speed, apparently the plan B will increase the cost of 4870 tremendously.