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, 3584 MB, 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 and see what power supply will suffice for it. You can see the results in the diagram:
The platforms with the junior Radeon HD 5670 and GeForce GT 240 are expectedly the most economical in this test session. They consume the same amount of power irrespective of load, so the new Radeon HD 5670 does not have any advantage over its market opponent here. I want to remind you that both cards lower their GPU and memory frequency in 2D mode and the Radeon HD 5670 also lowers its GPU voltage. Thus, both cards help save power when there is not much work for them to do.
The power consumption of two Radeon HD 5670 512MB in CrossFireX mode is comparable to that of the single Radeon HD 5770 1024MB. The difference between the results of the platforms with one and two Radeon HD 5670 indicates that the new card has a power draw of 55W or lower. It is the platform with GeForce GTS 250 1024MB that proves to be the most voracious. It needs 50W more power than the system with Radeon HD 5770. Well, what else could you expect from a graphics card based on an old 65nm GPU and which cannot drop its frequencies in 2D applications? Anyway, as you have just seen, a 400 or 450W power supply will suffice for a computer with one of these entry-level graphics cards.