I've been debating if I should upgrade to a different board/setup because at the moment my DFI board can't overclock(very far) with the multipliers and northbridge multiplier locked with cool and quiet enabled.
For max clocks on my INtel quads you would really want to disable speedstep for stability, but 98% of max clocks usually worked fine with speedstep enabled meaning far lower idle power.
Been slowly reducing my PC power usage, and was the main reason for swapping a 4870x2 for a 4890.
Some of the numbers, afaik, are rather incorrect though, a 4890 was supposed to use 60W at idle, but afaik because the memory couldn't be underclocked without the screen flashing it was never used in drivers so it really uses 80-90W.
If you enable your own profiles and set memory clock settings at minimum which is 500, down from 950ish my power usage drops 20-22W instantly. GPu clocks from stock to 240mhz, does nothing at all for reduced power. The other main power saving I assume from the new cards is reduced voltage on memory at lower clocks and also lower stable clocks on both memory and gpu helping also.
THing is, if you wait 3 months to buy a 5870, you'll save £50-80 on the cost of the card, and still get 3/4 of the savings in the next year for power, saving you an awful lot more overall.
Next gen should get even better as better features were enabled this time around, but no powergating tech like a current CPU will have. At idle, doing desktop work you could easily turn off all but one cluster of SP's aswell as only have them run at 150mhz, which would take power usage down towards intergrated graphics power levels.
Its going to happen purely because AMD/Intel are moving gpu's onto the cpu die, where they will need powergating options, which will simply crossover and be used in their discrete products also.