You don't need two loops, just one loop with two rads like how I've set mine up
Reservoir - pump - CPU - rad - GPU - Rad - Back to reservoir
In this configuration, neither major component need suffer from the heat of it's own generation AND heat from the other component without the benefit of the rad cooling the liquid before it reaches the other waterblock.
For 5 years now I have been using PC ICE, it's a bit pricey ($19.95/bottle and I use 1 1/2 bottle per filling) but it is a lubricant (for the pump), anti algaecide and it is non conductive. I can attest to it's truthfulness as my cooling loops have sprunk a leak or two and I have accidently spilled the coolant on active (powered) parts and nothing has happened.
Can't put a price on peace of mind.
I don't really advocate cooling other parts as the additional blocks only really serve to play havoc on the flowrate of a cooling loop. I would only consider liquid cooling the northbridge IF you do some really major o'clocking of your CPU/GPU. The northbridge coordinates activity between the CPU and the various PCI lanes (PCI, PCI-E, etc.) AND it also coordinates activity between the CPU, system cache (except on AMD machines) and the system memory. So, again, if you do some major o'clocking of those areas then, perhaps, you might consider cooling the NB. But, a good HSF can do a good job here.