The boys at ProCooling have a pretty good article up on flow rates... If
you're a novice water guy or want to see if you're maximizing your cooling performance
you'll want to read this!
From a technical standpoint, however, the limitation in the final performance
of all of my water cooling setups has been the pump. Centrifugal type hobby
pumps are just not designed to generate much hydraulic head, and a typical cooling
loop is pretty far from their original designed usage. To compensate, I try
to make the tubing and routing of the plumbing in my setups as low in flow resistance