water pump

where to put it in a water cooled system?

Totally up to you, but it depends on what your case is like. If you have your pump in a reservoir you might want to put it on the top(of the inside) of your case so that air will get trapped in the reservoir instead of the pipes, or if you have it in an in-line pump, it might be easier to put the pump on the bottom, unless your pump can't move water up to your cpu, lol, and have a extra pipe(hose) for an air trap.

Like CrackedSkull said, wherever you want to put it. I could only fit mine at the bottom of my case, so that's where it went. I know people like to try and keep them away from the rest of the computer, hard drives in particular, because of the powerful magnetic field they create (assuming it's a magnetic impeller type). When thinking about the max height the pump can move water you need to consider the total height of the watercooling system, not just the height above the pump. From a point of view of lifting water, it doesn't matter where the pump is, just as long as the water doesn't have to move too far overall. For a mid-tower case you probably couldn't make a watercooling system higher than a couple of feet anyway, so it shouldn't be an issue. Just cos I like showing it off, here's a pic of my watercooling