The test system can be seen on a page on my website. I performed the measurements running Prime95 with the small FFT size so that components other than the CPU are affected as little as possible. I used RMClock to vary the voltage. All tests were run at the default frequency of 2.1GHz. The raw numbers were:
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V/V P/W 1.1 98 1.2 102 1.3 108 1.375 113
Plotting P against V^2 gives a fairly straight line, as hoped:
I've shown the graph extrapolated down all the way to 0 voltage. There's no telling if the graph would actually continue straight like that, it's just there to show the intercept of the best-fit line.
The equation for the best-fit line has handily been calculated for me (there was a time when doing this would have involved actual maths - now it's all automatic). The number of interest here is 71 which is approximately the contribution to power consumption of everything apart from the CPU. The actual CPU power consumption is then 27W at 1.1V, rising to 42W at 1.375V. These numbers are in line with what I would expect given where my A64 X2 4000+ lies within the range of CPUs given a 65W TDP by AMD.
Note that the power consumption figures all include the inefficiency of the ATX PSU and the CPU's voltage regulator. If both of those were 85% efficient, then actual power consumption would be 0.7x lower.
EDIT: I forgot to say that I only ran one copy of Prime95, so only the FPU of one core was under load. I suppose I should have loaded both cores.