I had a Thoroughbred AthlonXP running at 2300MHz (200MHz *11.5) cooled by 20ft of 3/4th copper pipe that I ran along a wall behind my desk. Under load my cpu was 45-49c. The pipes (and water) were noticeably warm. I have since changed my cpu to a Barton at the same speed, and use a large single pass heatercore in a box with a 120mm fan at 5v. Under load the barton now stays under 38c. Passive cooling is nice because you don't have to worry about dust and is dead silent, but to get good performance it has to be HUGE or be in a cool place (which miami is not
). The 15 $us heater core I am using now performs MUCH better, is cheaper and smaller, and a panaflo fan at 5v is nearly silent.
Here are some pics of the passive setup I had: