WestWarrior is dead on....
Your approach to o'clocking is every bit as important as what components you have that you want cooled. If you are an aggressive o'clocker (CPU, GPU, Ram, voltage adjustments, etc.) than the price of a complete loop (or loops depending if you want two) goes up. This would be due to upgrades, or additional equipment (like adding a NB waterblock). The Northbridge, typically, controls memory functions like – a memory controller (for Intel Chipsets), a level 2 cache communicator and bridges the gap between the CPU and Ram – it also handles functions between the CPU and the graphics processor on the PCI, AGP and PCIe slots. Since this particular part is always busy it can generate quite a lot of heat.
Otherwise, if you just mildly o'clock than things like a NB weaterblock aren't as necessary and the NB can be cooled by proper airflow and a decent HSF.
Also, you have to decide if you want to go cheap (as in cheap parts - which I wouldn't recommend) or make that significant initial investment. After that, maintaining a watercooled system is very cheap and the nice thing about them is that they can typically grow with your rig as it changes over the years - with minor investments ever so often for an upgrade or two.
So, it really starts with your your o'clocking intentions. To go top-of-the-line with a rig that has a quad-core and SLI could cost as much as $400 (i.e D-tek Fuzion or EK Supreme, Thermochill rads, one or two pumps, nonconductive coolant, tygon tubing, Swiftech MCW60's, etc.)