A direct drive pump is not good in a computer application IMO. Those things have seals to wear out and some can leak oil or other stuff into your water.
I'd suggest a Danner MagDrive Model 7 - 700gph rating. You can get them from
PetsMart.com, plus as a new customer you can get $10 off your purchase!
I've got a Danner MagDrive 5 which is rated at 500 gph and can pump over 10' high. Haven't put together my whole system, but with the Aquastealth Aquacoil radiator(similar to the Cooling Cube but with higher flow) and about 3 feet of 1/2" ID tubing the pump was really flowing alot of water. I'll put it this way, it was shooting a 1/2" diameter stream of water over a foot and a half out of the tube when held level, that's after going through the radiator.
I think the Model 7 would be perfect for your setup. They're extremely quiet, submersible or inline capable, and pretty affordable.
As for your design, have you thought about going to a 5/8" ID barb for the input/outlet barbs on the distribution blocks? Might really help the overall flow in your system.