There comes a point in PC Cooling where air and closed-loop coolers just won't cut it anymore. The heat starts affecting thermal throttling, and the noise becomes too much. This is where custom water cooling comes in.
Custom Water Cooling is essentially the same technology in your car. A water pump forces water through blocks/plates that carry the heat energy of the core components away from those components. The flow continues to radiators that use powered fans to dissipate that energy into the air. Case fans will exhaust the hot air outside the case. The flow ends at the reservoir, which is typically placed directly over the pump, to provide the pump with a steady stream of water. Pumps tend to die when water is not running through them, as the water acts as a lubricant. Pumps spin very fast, and can overheat very quickly when there is no water running through them.
Temperature and Noise are the two main reasons to watercool. These are directly tied to the most important WC component..the radiator. To drop temperature, you could use more powerful fans, but those fans also run faster, which means more noise. To keep noise down, you can use a bigger radiator.
Bigger radiators can mean more fans/inch (FPI), however, that ends up creating more pressure needed to get the cooling air through the radiator. This can be alleviated with better/faster fans. However, the better method is generally to use a radiator with another fan slot on it. More/bigger fans means each fan can run slower, while maintaining the cooling capacity. There's a reason why 230mm fans exist. Some of those big fans can run under 1000RPM, while putting out over 140CFM. That's a lot of air! (by comparison, an standard 120mm fan will peak at around 2400RPM, while putting out about 62CFM, and is 10dB louder).
More radiators can lead to more powerful pumps/more pumps. If you are still experiencing heat issues, try splitting the loops up. Have a dual/triple fan radiator for the graphic card(s), and a single/dual fan radiator for the cpu. You can also include some other minor areas in the loop as well (south bridge, mosfets, ram, drives), although these tend to just add heat, as the do not typically need active water cooling.
There are a ridiculous amount of configurations and options in custom water cooling. If you would like something discussed in more depth, please email me at firstname.lastname@example.org