The Coolermaster Hyper48 series is supposed to be an awesome heatsink for the socket 939 mobos, actually it's compatible with P4's, and K8's. Ag3 has it and reported 10C decrease in putting it in. I looked into it and it does indeed look quite impressive. You can check it out here:
Great. I got my mobo and cpu today, too - unfortunately they're the only parts I've got! I look forward to hearing about how that heatsink works, though.