use a kill-a-watt since the power usage at the wall is what's interesting, with 7xxx series cards you can test with just one card since they pull around 3-5w at idle with the screen turned off.
For other cards what I like to do is testing a rig with 1 card first to see how much it draws, then add a second card and check again to see how much the new card draws. Another reason for testing with more than just 1 card is that when you have "low loads" like 1 card + cpu/mobo you can get higher numbers than you should due to PSU efficiency normally falling the lower the load is (peak efficiency for a PSU is normally in the 30-70% load area)
You should do the second part with 7970 as well. The ZeroCorePower only works if you only have DisplayPort monitors plugged in. Also ZeroCorePower seems to behavior quite oddly when you change voltage or clock settings.