I read the link above:
Ask A Scientist© Chemistry Archive Distilled vs Deionized Water
And here's my take, with bonuses from my college training:
Distilled water is what you want.
Distilled water is what you put in cars. De-ionized water is used as a substitue for distilled water because distilled water is comparatively expensive and not always necessary when ionically neutral water is needed.
De-ionizing water removes impurities (specifically, polyvalent ions) through a filtration process, but sometimes leaves or even adds different impurities (like Sodium ions) into the water. De-ionizing systems are sometimes called "softening" systems. Distillation is typically accomplished by boiling off water and collecting the vapor. Impurities in the water do not undergo a liquid->gas phase change (for metals, if you had alcohol in the water it would undergo a phase change, but at a much lower temperature), and hence are not collected.
Even distilled water naturally ionizes (if I remember my chemistry right, into H30+ and OH-, not H+ and OH-), and hence is conductive. There is no way to prevent this kind of ionization-- there's no such thing as non-conductive water.
The most important thing for water cooling is to insure that minerals like Mg and Ca don't build up in your water lines, cooling block, radiator, etc. So you want those impurities to not be there, not just to be ionically balanced with other impurities. Thus, distilled water is what you should put in your watercooling system.
(I kind of think it might make sense to use a non-conductive organic solvent, eliminating the water danger altogether. But the specific heat of low-viscosity organic solvents is probably too low to absorb sufficient heat out of the heatsinks without a really fast pump. Not to mention that the solvent might boil at 50C!)
Ah, the joys of a MA in Biology.