Well, here's my $.02. I'm a mechanical engineer, but I'm not a thermal or hydraulic specialist, so others may have more appropriate comments.

Pumps are rated by pressure and flow rate. You don't need a very heavy duty pump, because you won't need a high flow rate, and you will not be lifting the water very far (maybe just a couple of feet, if that). The horizontal run of the coolant doesn't matter (within reason, with a really long run you start to factor in pressure losses due to the run), what matters is whether you are *lifting* the water -- that means pressure is required. You are just moving water in this application, with a relatively short run and without lifiting it much (if at all).

If you are going to roll your own, I would recommend the quietest fish tank pump you can find. Get a pump for a larger fish tank, as it will probably have lines that are easily adapted to the size you will be using on your computer.

When you are dealing with cooling, the thing you want to do is dissipate *power*. Knowing how many watts your CPU(s) consumes (when overclocked) can give you an upper limit of how much heat you need to dissipate (to be conservative, assume that all the electrical power is turned into heat). Convert the watts into something you can use to size your radiator pipe, like BTU/hour.

What you want is a system that dissipates heat at at least the same rate (or faster) than the CPU generates it. In my humble opinion, since CPUs are generally just fine to +50C, there is no reason to use active cooling. If you can cool it to room temperature, that is plenty good enough. If you can keep the cooling block to room temperature, or even somewhat above, you're fine. This eliminates the need for active cooling and prevents any condensation/dew point issues.

You *will*, however, need to move coolant through the system fast enough to keep the cooling block cool. You will also need to cool the coolant, which is where the radiator business comes in.

You pretty much have 3 options for cooling the coolant: passive convection, active convection (using a fan), and some kind of refrigeration. Its really a question of surface area. Those 3 options are arranged in decreasing surface area requirement. Completely passive is just fine, doesn't require any additional power, and is silent, but it is going to require a lot more surface area than if you blow a fan over it, or if you use some kind of refrigerant.

So, for cheap, easy and silent, I would use a long length of tubing with fins on it (like the home heating radiator you suggested). To test things, stick your little CPU cooler on a bright, halogen light bulb (100W+) and measure the temperature of the coolant. You need enough radiator and fast enough flow rate to dissipate the heat.

So, that's my $.02. Cooling is a function of coolant flow rate and the size of the radiator. Of course, the heat transfer rate of the little thing you stick on the CPU is a factor, but I'm assuming this is a constant. From a practical point of view, it would be really easy to build a radiator that is "overkill" for the application. Then, the faster you pump the water, the closer to room temperature you'll be, limited only by the CPU cooler's ability to conduct heat away from the CPU.

FWIW,

Jim