Sorry but would somebody tell me how water cooling works simply. I'm guessing that it runs water through the heatsink, but still don't know how this could work.
The important thing with liquid cooling is that it transfers heat.
The liquid starts in a reservoir with a pump. The pump pushes the liquid into a "block" installed on top of the CPU. The heat is transferred from the CPU into the liquid. The hot liquid is then pushed into a radiator. The heat is transferred from the liquid into the radiator grills. The radiator has a fan that pulls the heat off the grills and into the air. The cooled liquid is then pushed back into the reservoir.
This site shows it in a pretty simple to understand diagram:
Google Image Result for http://img.webme.com/pic/g/guide-pc-cooling/02.gif
thats a good start of an explaination
the biggest reason for going with a liquid cooling loop
is the fact that the liquid (distilled water, anti-freeze[controversial], mineral oil, etc.) will absorb the sudden shock of heat that is generated by your CPU when it goes from idle to 3 cores running at max when you start that game
where as a HS&F has a very hard time keeping up with the heat load that has been generated
which is why you can hear your fans kick in at times
i have seen several rigs that have massive radiators
there are even some rigs out there that can run passive
(as in no fans required) check out the attached pic
Do you have to replace the liquid ?? Or can you keep it the same
yes, you should replace the liquid on a custom built loop every once and a while
on a Corsair Hydro
no, they are closed and should stay that way
it would be a PITA to try and close after you open it
although i have seen it done
and i have also seen a Corsair Hydro used to cool off a GPU