Buy you about 10 feet of tubing, it gives you some extra in case you screw up on cutting the length right or change your mind later on the way you routed the tubing or decide to add nb or gpu cooling later. At DD and D-Tek, they are talking about the tubing's ID size, not the fitting's ID size on the radiator or wb. But the 1/2" fittings on the rad are made to fit the 1/2" tubing. I ended up using about 6-7 feet of the 10 feet of tubing I bought, but I'm cooling 2 cpu's.

BTW, go with the 1/2" tubing and fittings on everything because you will have much less pressure loss in your cooling system than using 3/8" tubing and fittings. This in turn puts less back pressure on the seals in your waterpump and will improve the life of them, plus you will get better flow rate through the 1/2" tubing. This is due to the larger ID of the tubing giving lower flow velocities, which, helps keep the flow regime in laminar flow, which has the least resistance.You want to have laminar flow throughout your system except for the waterblock and radiator, which by their design should mechanically induce turbulent flow for better heat exchange. The only stuff I would consider 3/8 for would be to use in cooling the gpu or nb, which don't need the flow rate like the cpu would.

Part of my job is dealing with the flow of newtonian(water) and non-newtonian(drilling fluids) fluids, so I know what I'm talking about here.