[billbartuska]

OK, now I'm confused. How are you going to use the peltier to cool a radiator?
Most people that do what you are trying to do (use a peltier to cool the water) attach the peltier to the reservoir, ...which you could mount outside of the case.

as for the calculations, while appearing to be simple unit conversions (CPU watts to *C of the water), they are compounded by the fact that the peltiers cooling is constant, but the heat generated by the CPU varies with load. You may want to make sure that at the lowest CPU load, the peltier won't reduce the water's temperature to below the dewpoint of the ambient air, or condensation will start to appear, and, as we all know, water + electronics = $$$$. Of course insulation is an option. Then you are only limited by the ability of the pump to continue to move the ever thickening (as it cools) liquid.

Soooooo You have to decide what you want to do before you can start figuring out how to get there.
What CPU temp at what CPU load are you shooting for?

I don't think you understand how a peltier works. When you apply current to a peltier one side gets cold, but the other side gets hot. In order for the peltier to function as you envision it (cool the radiator), you have to provide a means for removing the heat from the hot side of the peltier. That is usually done with a heatsink (with or without a fan) or with a waterblock and water cooling system.