[Ewan]

You seem to be assuming that flow and head are two distinct characteristics. They are not. One (head) is a function of the other (flowrate) and can therefore be discarded.

Flowrate is the only measurement you need to worry about since it is what does the cooling. Whatever pressure the water is at, is completely unimportant.

Head means pressure and it's a stupid word.
Liquid flow through a system is motivated by a pressure difference. i.e. it's the pressure difference that makes the liquid flow form the high pressure place to the low pressure place. In a closed system the flow will go from the pump output to the pump input because the pressure at the pumps output is higher than at it's input.
Because of friction between the liquid and the pipe walls as the liquid flows through the pipe, there is always a certain resistance to the flow which will slow it down. This resistance is expressed as a pressure drop, i.e. the pressure differential you need from one end to the other to get the fluid to flow at a certain speed through the pipe.
The pressure drop gets worse if you have kinks and other obstacles which the fluid must negotiate as it goes through the pipe.
Pressure drop also gets worse as the flowrate increases, naturally because the frction force will be greater, because the fluid is rushing past the walls faster.
A watercooling pump is almost always a centrifugal pump, so it will pump as hard as it can to overcome the system pressure drop. So the flow of the fluid will be at a point where the system pressure drop is equal to the pressure that the pump is providing. The higher the flow, the less pressure it is able to provide, so an equilibrium is reached between the pump pressure and the system pressure drop. i.e. the flowrate will be such that the two are equal.
But the system pressure drop is not dependant on the pump, only the flowrate. Which means that if you put a pump which is capable of higher pressures, then the flowrate will increase so that the system pressure drop (which increases when you increase the flowrate) increases to match the pressure being provided by the more powerful pump.

Having said that, most water cooling pumps provide as much flow as their presure will allow them, so getting a pump with better "head" is generally the way to go. This is because watercooling pumps have such pitiful pressure (often less than 2m of water) that they are unlikely to ever put out their peak flow.

Another type of pump is the diaphram pump which will always put out a specific flowrate because it uses pistons to meter out a certain volume of liquid with each stroke. Such a pump will always produce the same flowrate regardless of the pressure drop that that flowrate generates, unless the pressure drop is so high that the pistons aren't able to provide that pressure, in which case the pump simply wont work. Such a pump would have no probelm with whatever restrictions were in a watercooling loop since they can put out pressure of well over 50m of water and in such a case you would get the one with the highest flowrate.

As I said at the start, the water pressure is not important to the water cooling, only flowrate. However flowrate is the same throughout the loop, whereas the water pressure will be greatest coming out of the pump and the least going into the pump. Where you put the pump in relation to everything esle is for this reason not important since the flowrate will be the same regardless of where you have the pump.

However heat transfer (which is the final goal) is helped by temperature difference between the water the whatever it is you transfering heat to/from. This means that the colder the water going into the CPU waterblock is, the better the cooling. Obviously the water will be coldest coming out of the radiator so the radiator should be positioned infront of the CPU waterblock. By the same token the water will be the hottest after it's gone though all the heating elements. That being all your waterblocks and your pump (the pump itself will add some heat). The water should enter the radiator when it at it's hottest so that it will be able to remove as much heat as possible. So the radiator should be positioned after the pump. Like this:
Pump -> Radiator -> CPU block -> other blocks -> back to the pump.
If you have two pumps have them right next to each other in series before the radiator. If you have them in a push-pull configuration then it simply means that you are adding heat from the 2nd pump to the water after the CPU block before the water goes to your other blocks. This means that the other water blocks wont work as efficiently becasue the water will be warmer.
Having two pumps in series doubles the pressure which should provide a roughly 40% increase in flowrate.