FUD Zilla reports the RV790 is an improved version of the 55nm RV770, with better power efficiency. The Radeon HD 4800 series consumed a lot of power in idle, and while the Radeon HD 4890 will use more at full load than its predecessors, the idle consumption should be lower.
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.