Checkout the reviews of coolers at http://www.silentpcreview.com/
The Scythe Ninja and the Thermalright HR-01 have the ability to cool passively but I only recommend it if your case has good airflow. The Ninja is hard to find but a revision "B" of it has just been release on 10/2/06 and should enter circulation soon.
The Scythe Mine would be a great active cooling solution but it will probably be a bit more than $40 (U.S.) after shipping.
Then there's my favorite, the Arctic Cooling Freezer 64 Pro which can be had for under $30 (U.S.)[/plug] It's not as quiet as the Scythe Mine, but it's a lot quieter than the stock AMD heatsink.
Word of caution. All of these heatsinks are quite tall, so check to make sure they'll fit in your case before ordering.