[billbartuska]

You're right. In series the 1/4" GPU block will restrict the flow to the CPU block. But, in parallel the 1/2" CPU block will get most of the water.

The solution isn't a bigger pump (though that almost always is a positive).

The solution is a new GPU block which will function efficiently (ie. cool the most) with the flows you anticipate with the new pump.

Ya kinda have to look at pump curves and block restriction curves and see where you're at and where you want to go. Don't forget to figure in pump losses for tubing, fittings, rad, and ras

As i see it, parallel/serial isn't the question.