quad core Opteron processors from AMD draws less power than the current dual core Opterons and AMD is already bragging about it.
During the AMD press conference at Computex today, a demo of the new Opteron's showed them as being much more power efficent than the current dual core processors.
AMD had set up what was apparently two identical systems, one with two dual core Opteron's and one with two quad core Opteron's. The quad core system drew 214W at full load, while the dual core system drew 259W. This varied slightly, but overall the quad core system was some 50-55W more power efficent.
In reality this means that you get twice the computing power in your servers without having to worry about any increase in power, which will make a lot of people happy, as there are already problems in most server rooms with new servers drawing way too much power.
Here's a picture from the live demo that show the dual core system on the left and the quad core on the right. Note that it's the first meter that shows the Wattage.