how many watts is the fan? from what ive read boards are supposed to be able to handle around 4w without the 3pin header taking a "header" (sorry couldnt help it)
but still anything over 2w i would be very wary of plugging into the mobo
by the way if the fan isnt rated in watts, but is rated in amps just multiply the amp by the volts (in this case 12v) and it will give you the watts