Well it was early...
Like I said, the pump is fine. I'll be trying a couple of Vias (maybe even more than two!) in that configuration, because I finally figured out that the pumps we need should have a good head, and that's the way to achieve it with centrifugal pumps. I'm shooting for 300 gph (but hey, that's just me!). Also, the Vias are pretty cheap, at about $20 each.
I don't like silicone tubing. It is good because it is very flexible, but this porous aspect will cause it to retain the color of an additive, as well as sweat out (albeit minimally) some coolant. Since I'd rather have a sealed rig (so I don't smell the additive), and since I'll be adding UV dye, vinyl is a better choice, because it is clear. It would also let me see if there's any junk building up in the coolant.
The flow sensor is a luxury, because it really doesn't do anything very usefull. BTW, i wasn't talking about a flow meter, just a flow sensor: something that would indicate wether there is flow or not. If there is no flow, for whatever reason (pump failure, power short,...), it would be nice to know, and since we don't always sit at our computers, an alarm would be an extra luxury. The ultimate failsafe however is the CPU diode, with a built-in reader on the mobo (or added, see pHaestus' article) that shuts down the computer. Note: some add-ons rely on software, so if it's not running, then there is no protection.
As for the pump wiring, I think that since everything should fit in the computer box, there is no need to have two power cables coming out of it. As for turning it on and off all the time, the coolant would heat up if the pump alone is left on, unless a fan is still on, and that's not easy to do. Personally, I'd rather shut the whole thing off, but I guess I'll have to see for myself what happens when I do that.
Enjoy, and keep us posted!
(thanks to Skulemate for pointing out what a PC75 is!)