Interestingly when overclocking a single GeForce GTX 460 card (175MHz core/220MHz memory) we see almost no difference in power consumption. Adding a second GTX 460 board increased power consumption by 48% which is a solid result considering that on average this configuration provided 75% more performance.
The GeForce GTX 460 SLI graphics cards used just 141 watts at idle and 447 watts when stressed in FurMark. To put this into perspective, the SLI cards consumed about the same amount of power at idle than other single boards that were slower.
The load results also look favorable on a performance vs. power consumption ratio. The GTX 460 SLI boards used 5% more power than the GeForce GTX 480 and 32% more power than the Radeon 5870, however on average they were 16% and 43% faster, respectively.
Compared to the dual GPU Radeon HD 5970, they are a close match in terms of efficiency but at a fraction of the cost.