Wow it's a tougher question that one would think.
Actually 600 l/h is a lot already. In your case i dont think putting the blocks in parallel (with 2 Y's) would yield far better results than putting them in series. Actually water temps dont increase that much (generally no more than 1 or 2°C), so the NB wont fry. So go the easy way, Pump->rad->cpu->NB->pump all in series, make sure you've got a minimum of turns and you'll be OK.
On the other hand if splitting with a Y give you *less* turns (ie no need for elbow fittings for example) go that way (parallel):
Code:
pump->rad->(Y)->CPU->(Y)->pump
\->NB->/
The major flow killer is turns and elbows.
In short, do what's easier and cleaner for you. It will reflect magically on the performance.
Ah the 1048 is quite enough for most 'usual' applications. Just glue that impeller
(edit) Hehe hmale beat me to it