^^^ That completely depends on why someone is going watercooling as the reason you want to and the layout you plan (inside case or outside etc) all effect your choice of rads, number of rads, fan speeds etc. If you want almost silent running you need low FPI (fins per inch) meaning low speed fans but then cooling isn't as good as high fpi and higher rpm fans.
Taking me as an example, I am looking at watercooling for better cooling and for even quieter running. I want to keep it all contained in my case which means 120.3 in the roof. Picked SR-1 rad as low FPI (9 from memory.) Looking up delta tables for my cpu and gpu at full load according to calcs I will need to go disapate at max 400 - 450 watt of heat (calculated on 4Ghz overclock) meaning my fans would have to run at about 1200 rpm or so. Thats too fast for what I want so back to keeping in my case, I can add a 120 rad on the rear which should mean my fan speed even at full load would be ~1000 rpm or lower. Bear in mind that all this is calculated at 100% load on cpu and gpu which in the real world is never going to happen so I should be able to run my fans really low speed for general use like internet and films.
I would suggest the OP runs the same methodology though obviously it will change depending on what you want your watercooling for - silent/almost silent running, ultimate cooling with no care for noise or somewhere in the middle etc all effect what rads and fans you get.
