Of primary concern in any potential liquid cooling loop should be the CPu and then the GPU - in that order. Cooling much else is, typically, more a personal preference and is not actually a necessity.
Cooling the northbridge is just a novelty unless you do some heavy o'clocking. You see, the NB handles communications between the CPU and the various PCI lanes (i.e PCIe, PCI, etc). and it also handles communications between the CPU and the CPU cache and your system memory - for Intel machines. AMDs proc have the memory controller on the CPU so that lessens the CPU load. If you don't do much o'clocking than a decent HSF will suffice. Cooling your memory - I'd never recommend that, even if you o'clock. As long as you have decent airflow within your computer that will halep with keep the ram cool. Putting ram in a cooling loop will just cause the flowrate to commit suicide and diminish your cooling loops efficiency.