to put the fan on the mosfet heatsink.... THe heat emitted from the MCH (northbridge) is transferred to the mos fet sink via the copper pipe and then blown away. The fan that comes with the board cools them both.
On a side note, I miss my P5W DH Deluxe
I hit 500 FSB on that piece, and could actually utilize my RAMs full potential. I probably should have kept that board, waited, and got Xfire with the new R600 cards... But oh well.
Hope this helps.