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No fear... 2x 480 should be sufficient for 6x gpu + 1x cpu + 120mm - without overclocking.
The discussion expanded thru heat dissipation details and power consumption.

should be sufficient for 6x gpu + 1x cpu + 120mm

What is 120mm here?

2x 480mm Radiators (8x 120mm) = 6x gpu + 1x cpu + 120mm

I see, you are referring to the 'one component = one 120mm fan' rule?

I guess it's simple enough for newbs, but there is a huge difference between a high fin density radiator with 3000 rpm fans and a low fin density radiator with 1000 rpm fans.

Correct, that's what i wrote yesterday, that it is a rule of thumb. 120mm per non OCed component + 120mm per OC.