I measured the power consumption of systems with different graphics cards using a special power supply. To create maximum load I launched FurMark 1.7.0 in stability check mode at 1920x1200 and Linpack x64 (LinX 0.6.3, 3500MB, 7 threads). These two programs load heavily the graphics card and CPU, respectively, so we can determine the peak power draw of the whole system.
You can see the results in the diagram:
The systems with CrossFireX configurations need more power than the systems with single-GPU top-end cards. Anyway, a 600W power supply is going to be sufficient for every tested system. A computer with a single Radeon HD 5770 or HD 5750 can do even with a 450W power supply, especially as such a computer is unlikely to have an Intel Core i7 overclocked to 4.1GHz.