The motherboards itself would barely use any power. It's everything you plug into the board.
GPU, CPU, fans, lights(bling), drives, ram, ect...
Typically you just go by what the GPU manufacturer recommends anyway. Lets say your plan is to get a 7970 (GHz edition). A quick check at the AMD website... recommended PSU = 500 watts.
The 7970 itself would only use ~250 watts (max), plus about 77 to 95 for an Intel i7, so total being about 330 watts. The rest of the draw would be minimal, no where near the remaining 170, but you never want to load a PSU right up to it's max.