The Inq reports the new RV670 graphics cards will dynamically change their GPU and memory clockspeeds to lower power consumption. This snip of information was found in the changelog of RivaTuner 2.06:
Hardware monitoring module now properly detects DPM (Dynamic Power Management), a key feature of upcoming RV670 chips. Riva Tuner can monitor dynamically changing RV670 GPU and memory clock frequencies, adjusted in real-time.
As far as we know, RV670 chips will change their clock and power consumption according to 3D load. So, if you're running a highly demanding scene in a game, the GPU will be clocked at maximum, speed, while running cut-scenes or any lower-GPU intensive scene will cut down the clock of the card and decrease the power consumption.
This is something that Nvidia cannot offer with its GeForce 8800GT, at least according to Daamit's marketing slides.