Power usages for me look like this:

Max formula with q6600 g0 @3.6 folding 2*smp (2665's = 3600ppd approx) gpu fans @100%
watts = 440

add 1* GPU = 500watts (+60watts 380pts wu)
add 1* GPU = 560watts (+60watts 380pts wu)
add 1* GPU = 615watts (+55watts 353pts wu)
add 1* GPU = 680watts (+65watts 511pts wu)

changing the shaderclocks changes the power consumption some too (adding 54Mhz adds maybe 3 watts and up to 2 degrees C) but this should give a fairly clear picture.

My Kill-a-Watt clone is not the most precise tool