In a single-pass radiator, the coolant flows from the inlet at one end, straight through all the tubes all at once, to the outlet. Single-pass benefits because it presents all the tubes with the highest possible water temperature at once (whereas a two-pass radiator will only get the highest temperature inlet water on one side, and then cool slightly cooler water up the other side. This doesn't make a huge difference though. Generally it provides a 1-15% performance benefit from this effect alone).
Where single pass falls down, though, is the tubing velocity. Because the water is presented to all the tubes at once, the water velocity through the tubes is half of what it is through a dual-pass radiator. As the flow rate (and hence water velocity in the tubes) goes down, the radiator performance starts to fall away. With a single-pass you've gone and halved the water velocity in one hit. This is offset somewhat by the temperature delta benefit of single-pass, but it is by no means a sure thing that single pass will be better.
In a dual-pass radiator the water flows down one half-side, U-turns, and back up the other half-side. For multiple row cores, dual-pass is always better and flow restriction doesn't really come into it. The vast bulk of the flow restriction in these types of cores all comes from the fittings. The pressure drop difference is insignificant when water blocks are involved, so long as the radiators have correctly designed end-tanks. Higher CFM fans are more suited for this type of radiator.
The D-Tek Fuzion has a nozzle kit specifically designed for "quad-core" CPUs - the EK Supreme and the Apogee GTX are also designed with that in mind (so don't feel that you are limited to one choice).
While the Thermochill rads are considered by many to be the top of the line, their price/performance ratio might be a little steep - the Black Ice Xtreme 2 series of rads are excellent performers and would do just fine - I use two (2) Black Ice Xtreme 2 rads in my cooling loop (check out my sig).
As for tubing - ideally, 1/2 ID is probably the most popular. There are many different brands (of which Tygon is considered top-of-the-line). Just make sure that you get at least 10ft. That is a bit excessive but it will allow you to make corrections if you misjudge tubing lengths).
Also, consider teflon tape. Teflon tape is an extremely lightweight tape that plumbers use around threading. You would wrap it around the fittings before you screw them onto waterblocks and/or reservoirs to create an airtight seal to prevent leakage.