Me personally will all ways go with a 2 loop system if using a High end GPU system. Its best to separate them so that you as much more performance over the GPU and the CPU.
Many people will give you advice over what to use how ever not ever one is right. The best thing to do is start with a single loop see how things go and then if you can afford it move to a two loop system. the biggest problems is price. The second biggest problem is being able to house such a system.
if you use this rad you could use two pumps and have the line split into 2 single lines one for the GPU and one for the CPU then when they merge and go back into the rad the liquid will be cooled down more effectively.
here is a pic i just made for you