Yes, 1x 240mmx120mm radiator will be enough for your CPU and GPU.
I would recommend setting your loop up CPU => GPU => Pump => Radiator => Reservoir. If you can get the radiator mounted on the top of your case where the 2x 120mm fans currently sit, blowing the hot air out, you will have a very nice cooling setup!
------------------------------If the automobile had followed the same development cycle as the computer, a Rolls-Royce today would cost $100, get a million miles to the gallon, and explode once a year, killing everyone inside.
PSA
Reply to outlw6669
Only one radiator will be enough.
As for the loop, I would suggest inverting the pump and radiator, resulting in the following sequence: PUMP=>CPU=>GPU=>RADIATOR=>RESERVOIR=>PUMP.
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