I have been through many different PC's and loop setup since i went into this 10 years ago, parallel, dual, etc. For my most recent build i finally decided on a dual loop:
- CPU + GPU's in parallel
- Motherboard
For the only reason that the motherboard full cover block is so restrictive, it kills the flow even with dual pumps. So i have dual pumps on the CPU+GPU loop, and a single pump on the motherboard loop.
Then the CPU loop has GPU's in parallel, and 3 radiators (2 triple 140 and 1 triple 120). So i can handle full load on everything, or turn off (completely) a lot of fans at idle. This makes a pretty silent PC in light use or idle, and it starts whooshing a bit at full load. And on medium loads (video encoding which is CPU only) it is still very silent thanks to the extra rads, which is the most important in regular use. The dual loop setup here was just to maintain enough flow (i have 1.4 Gpm / 310 lph in each loop) and convenience too, as changing my vid cards wont involve dismantling the motherboard loop, and tube routing is a lot easier.