Let's observe the total power consumption at the Power Supply Unit (adding the whole configuration and the losses of the PSU) with each card.
The enhancements made by AMD on the RV670 are interesting. While idle, the consumption of the Radeon HD 3870 X2 is quite a bit lower than a GeForce 8800 Ultra with almost 30 W gained in total. However, at peak, we are dangerously close to the record of the Radeon HD 2900 XT, though never breaking it. Overall, the performance - power consumption ratio isn't really in favor of the 3870 X2, as it's similar to that of the 8800 Ultra, despite its age. We could have done much worse however, given the nature of the card and the usually poor efficiency of multi GPU solutions.
Compared to the HD 3870, the power consumption increases by 82 W, once the PSU's losses are subtracted. We've also noted higher consumption peaks reaching 385 W in some games (e.g. STALKER). The value of 196 W given by AMD regarding the maximum power consumption of this HD 3870 X2 seems to fall far short and we recommend the use of a 450 W (branded!) power unit supply for a classic high end configuration, not too overclocked.
Regarding temperatures, and despite the fact that the heat sink remains extremely hot, the standard design was conceived to speed up at 78-81°C. We haven't gone beyond 81°C according to the drivers, without knowing which GPU was measured. In idle, the temperature dropped to 56°C, which is quite low for a GPU.