Are you talking about CPU temp? 20C? That's like 68F! How are you gonna do that? Maybe with a peltier or something. Yes, the colder the better. You have to think about condensation though. Water and electronics don't mix well together.
48-52C isn't that bad. If that's the only reason you're thinking about water cooling, I'd say don't even bother.
A resevoir isn't really necessary, but it does make it easier. One thing I never understood about my water cooler. It's a closed system, and yet every few weeks I have to add about a teaspoon of water. If it's a closed system, where the heck is the water going?!!

So, if you have a big resevoir, than you don't have to worry about adding water as much (if it does dissappear like mine does). One good option is to buy a submersible pump and put it in a tupperware container. That takes care of your resevoir problem.