The "Series or Parallel" question usually boils down to your tube size/block restriction vs. your pump power. You should test both if you can to see what your flow and temps are and what works best. Stay AWAY from using "T" fittings. Use "Y" fittings only.
My current pump is overpowered for the 3/8" ID tube and high restriction on my GPU block. I just swapped out the CPU block with a new "experiment" that flows really well. I've also added in a HDD block which has good flow.
So, here's what I did.
I put the CPU on one parallel circuit from the raidiator and the GPU & HDD cooler on the other parallel circuit from the radiator.
They meet up just before returning to the reservior. Here's a picture before I added in the HDD block. The GPU is in parallel with the CPU. My temps are fine and my flow is good.
