DarkDraco
First, you are doing the right thing by looking at the forums for this info.
In a water cooling setup every choice you make affects the performance of your system.
For instance, a person could have a high-flow capable waterblock on the CPU, and another high-flow block on the GPU,but the Hard Drive block might take the little 8MM hose instead of 1/2 ID or 3/8 ID of the other blocks.
In this scenario it would not make sense for you to run your loop in series, as the hard drive part would be really constrictive and would negatively affect the flow at your other waterblocks.
So in this case you might want to consider building/finding a manifold that would take the flow of the pump and split part of it to go to the CPU, another part to the GPU, and another to the HD. Then re-combine the flows after they have picked up heat at each of their respective exchanges and send it to the radiator for cooling.
But if you split your flow three ways, you might now need to choose a pump that can move much more water than before, because now the velocities at each of those three blocks has been cut to less than 1/2.
It is my opinion that although the concept of HD cooling seems attractive, one might just want to make sure they use a fluid-bearing HD instead and still get a quiet system. Water cooling a HD yields Zero performance advantage there.
Your water cooling will most benefit (In order) your CPU, your GPU, then your North Bridge.
System memory, video memory, and hard drives benefit little in comparison to the above three by watercooling.
But I have to admit I admire some fanless systems out there where they watercool EVERYTHING, including RAM, Power Supply, HD, etc. A lot of effort and leakpoints though...