simple way to calculate how much radiator you need is take the total power draw of your system @ load.
That means both cpu and GPU.
If you dont know that, then take the TDP on the gpu and double the TDP on the cpu to account for Overclocking.
Add all those numbers up and you get a Value of X.
X / 150 = Y
Y = to the number of 120x1 surface area you need to maintain a decient delta from ambient to watercooling temp.
Then increase Y+1 for slow speed fans... or keep the value Y for medium fans.
So example is u got a 480 = 120x4 =
Y = 4
150x4 = 600W your good for on that 120x4, with medium powered fans.
You would have 150x3 = 450W of cooling potential on Slow powered fans.
Does this make sense?
There is a lot more in the mix.. like flow rate.... but that was why i was asking you, are you after xtreme LC temps on the cpu?
Your cpu will not be as cold as someone who has a dedicated cpu loop.
But your gpu's will probably be almost the same as if u had them on there own loop.
Meaning a net reduction of 1/2 load temps ur currently seeing.