We assume that RV770 core frequency is not far from the core frequency in RV670. However, the quantity of some functional units in RV770 in comparison with RV670 will be increased, and high-end video card on base of RV770 will get the faster GDDR5 memory .
The TDP level for Radeon HD 4870 will be equal to 160 W, and for Radeon HD 4850 this value will be limited to 110 W. In this case the Radeon HD 4870 will be on average up to 30% faster than Radeon HD 4850.
First, the core frequency in Radeon HD 4870 will be 100 MHz higher. Second, the memory frequency in Radeon HD 4870 will be equal to 3920 MHz DDR, and the radeon HD 4850 memory will operate at 2000 MHz DDR frequency . The first video card will be equipped with 512 mb GDDR- 5 memory, but the second will be limited to 512 mb GDDR- 3 memory . If we believe the existing information, the Radeon HD 4870 will be up to 52% expensive than the Radeon HD 4850 ($349 against $229).
But if we compare the speed level of the new video card AMD with the geForce 9800, then Radeon HD 4870 will be up to 25% faster than GeForce 9800 GTX, and Radeon HD 4850 will be up to 25% faster than GeForce 9800 GT or GeForce 8800 GT.