Like Snerp has said above me, the best flow path usually goes Pump->Radiator->CPU Block->GPU Block->Miscellaneous->Reservoir->Pump. The idea is to have the coolest water possible, (Directly from the radiator, no heat added by the pump or other waterblocks), to go the most heat critical component, the CPU. After that, most people put the GPU block. Some put a chipset block before that, mainly because chipsets add very little heat to the loop (in comparison to the other components), mainly because it is easier to rout the tubes that way. After that, if you have them, miscellaneous blocks come in, such as HDD blocks and such. After that, the water goes to the reservoir, so that the pump can get air free water. Then it repeats itself.
agree with these guys and hence thats how mine is setup
allthough in all actuality the difference in temps may only be 1 or 2c but then again thats the difference between 1 or 2 c and when your setting it up if all you have to do is change the order stuff is lined up then why not do it (right)
before all cooled components as stated above gives the garunteed coolest water to the cooled components, your pump is more then likely water cooled so therefor it will ad some heat (although minimal) to the water
next becuase thats the one thing your usually pushing the limit of and when your pushing the limit a 2c difference can be just what you need however in all actuallity you could line it up to any block just put the one you want the coolest first and so on and so forth
now next wiould prob be the reservoir and there are 2 main reasons for this- one if you have it mounted at a higher point then the pump the weight of the water actual helps the pump and as stated above it helps to rid air from the system if theres any (although there shouldnt be unless you have a leak at the pump suction or something)
so yeah pump-rad-cpu-other blocks-reservoir