I'd use 10mm copper tubing for the NB & GPU barbs, The ID is the same as a 'comercial' 1/2 barb (9~10mm) but you can use 3/8inch tubing which fit's
much better!. The only downside is you'll have to make your own Y splitter that does 1/2 inch to 2x 3/8. I'm going to use 13mm & 10mm Cu tube to make mine cause plastic ones are VERY restrictive...
I'd have the GPU & NB running parallel after the CPU like this:
Code:
____NB_____
| |
__CPU_____| |_________RAD
| |
|____GPU____|
If you have a two outlet block (White Water) you can dispense with the first Y splitter, just make sure you balance the flow through the NB & GPU so they match headloss/restriction...
(20 edits later!!)