While this article, like ANY article that has ever been published, is not a be-all, end-all on the subject, I almost feel like crowing about it; I've been urging people to consider things like power usage and their real needs for years, before arbitrarily wasting their money. IFF (if and only if) it is important to you, this article is a good effort into dealing with a highly subjective can of wrigglies. Sure, it's loaded with disclaimers, but it is an excellent start.
It is temping to say "Wow, AMD cards sure use less power than nVidia cards," but that would miss the point, and a nVidia card is still what you should buy if its combination of features, power, and power use are right for you. It's like the bogus "CAFE" (Corporate Average Fuel Economy) that automakers use to disguise their [customers'] preference for gas guzzlers; in this case the HD6970 looks like a bad deal.