We’ve heard that ATI's secret weapon with RV790 is not actually the higher clock speed, but rather improved shader efficiency, which enables it to do things a bit faster than a RV770.

This would mean that the RV790, Radeon HD 4890 card should end up faster compared at the same clock and specification to HD 4870 card.

We are not sure what ATI did to its shaders, but this is the reason why RV790 chips are actually differ from the RV770, and that the pin count is not the same. The release day should be April 6th.