With each of the graphics cards installed and running, I used a watt meter to measure the power draw of our test systems. The monitor was plugged into a separate power source. The cards were tested at idle in the Windows desktop and under load while running our Doom 3 "heat haze" demo at 1280x1024 with 4X AA.
Note that our power consumption numbers aren't entirely comparable from card to card because we're testing entire systems, and those are based on three different motherboards. Two of those motherboards are nForce4 boards, but the third is an AGP system. Also, notice that I've limited our testing to actual products that we have on hand, to the exclusion of underclocked "simulated" cards. Some folks have pointed out that different models of graphics cards may vary with respect to the amount of voltage going to the graphics chip or memory.