Reffering to these two tests it's a quite big difference (the test system is all the same):
http://www.tweaktown.com/reviews/160...ard/index.html
http://www.tweaktown.com/reviews/157...ard/index.html
showing a difference of about 50 watts under load. In other reviews I saw similar results considering the ddr5 not to be as power saving as it should be.
But I guess I should really go for the 4850 to be a bit more on the safe side.