I think I have decided on 3/8, due to not wanting a lot of overall bulk in the system, for my plan 1/2 would be a lot to deal with after I gutted my case so I could work on it. . Seems as long as you have enough pump to overcome the higher restriction it really doesnt matter performance wise, and blocks top out after a certain flow rate regardless.
The system I am planning out so far will be:
DD TDX with High FLow 3/8
DD Acetal low prof Maze 4 GPU with High Flow 3/8
2x CSP-MAG in parallel 3/8
DD BIX 1/2
Tygon 3606 3/8
2x custom round reservoirs with DD hi flow barbs
On the following gear:
Abit AV8
AMD Athlon 64 3000+ WInchester
2x256 GeIL Platinum DDR500 (dual channel)
Sapphire Radeon 9800 pro 128
2x Hitachi 160gb SATA
other drives etc
and this is the case I am working with
here