+1 rdc85 Most people don't think twice about their PSU, and most OEM's use the cheapest unit with a high wattage "advertisement". They need to err on high side with room so spare to attempt to guarantee it will work without liability issues. In reality, unless it's a quality brand the usable safe wattage is about half of what's advertised, give or take 25%! This goes for both Nvidia and AMD.
well i have 500w fortron 82% psu, but its nearly 3 yo and i have a q9550 little oced. so with evga 660ftw i am getting 239watt usage from wall.
but 7970 has TDP of 230-250Watt (GHz edition) while 670 has 170Watt TDP, yet both have psu req. 500-550Watt
now i would like to purchase a 4gb version of 670 but these have 550Watt psu req. i guess i will have to wait and see how 770 will fare