I'm not sure but I think you have to register your motherboard in order to get an answer from tech support. Either way, it doesn't matter. I would put a Zalman NB47J on it with a 40mm coolermaster fan taken from a coolermaster heatsink. I put on the fan with some thin wire on the side of it.
One guy in general hardware actually copied and pasted an email from Asus telling him the Zalman passive could be a solution. I don't remember the exact wording. But, it's like they were trying to push him in that direction without explicitely recommending it. (Maybe because they don't want to appear to be doing advertising for another company or some odd legal reason.)
I find with the 40mm fan on it, the Zalman is cooler to the touch. Don't know exact temperature but it used to be pretty darn hot.