The new RV790 chip is a 55nm chip and it looks like we are talking about slightly modified and improved version of RV770 55nm chip. We already wrote that it has a different packaging and at least one pin difference.

The new chip has better thermals and it will have better idle power mode. Radeon HD 4890 should consume less energy in idle mode but due to its higher clocks, it will probably have higher power consumption in full 3D mode. 

Shader units should be a bit more efficient but we'd advise you not to expect big improvements. This is a chip to get ATI by until it launches is RV8x0 40nm performance chip and it should be enough to hold the fort until the new 40nm performance part is here.