Korndog said:

adding stuff to your system, regardless of how its added, will decrease flowrate.

Incorrect. When you have two things in series, the total resistance is greater than either one. When you run two things in parallel, the total resistance is less than the resistance of either one. The exact equations for 2 resistances in an electrical circuit are:

Series: R1 + R2 = Rtotal

Parallel: (R1*R2)/(R1+R2) = Rtotal

Do the math and you'll see that the total resistance is way lower in the parallel case. I'm sure that the equations for a fluid system are equivalent to these. Lowering the total resistance will increase flow rate for the system.

But, I wouldn't advise running anything in parallel with the CPU block. You get more flow in the system, but you get less flow in each individual branch. Think of it this way, if total flow in the system is 1 GPM, you can't have more than 1 GPM in either of the branches, unless one branch has negative flow rate (going the other direction). So, since you want as much water going through the CPU block as possible, I wouldn't put anything in parallel with the CPU block. Some do, but I wouldn't.

Another advantage of running two rads in parallel is that both rads are getting the hottest water in the system. We know that heat dissapation is fastest when the delta T is highest, so you want the hottest water to be going through both rads.

Korndog said:

heater core removes heat, obviously. if can't remove all the heat dumped into the water by componants by the first cycle, then obviously the Heatercore is inefficiant for the amount of wattage your system produces. If your Heatercore can remove the the heat from you water efficiantly a larger or a second heatercore will not remove more heat then the first.

This is true if the water temp at the outlet of a single radiator is the same as the air temp. However, this is hardly ever the case. The lower the delta T, the slower the heat dissapation. Need I explain more?