In order to go completely silent at 345 watts you would need to go to the RM850 which is silent up to 40% of its capacity or 340 watts, which is close enough.
The most quiet power supply possible would be one powerful enough to run your computer without its fans ever spinning up.
Your computer should consume about 345 watts of power.
The Corsair RM750M fans don't become necessary till 300 watts meaning it should be silent for your computer unless you are using your cpu and gpu at 100% usage at the same time. Worst case they would spin up to about a near silent 15 decibels.
In order to go completely silent at 345 watts you would need to go to the RM850 which is silent up to 40% of its capacity or 340 watts, which is close enough.
I dont think the op is expecting complete silence , just something better than an apevia 'box o crap' which he should never have paired with those decent components.
You only really need a 550w psu , id gk with a 650w gold with semi fanless mode justbfor the exyra overhead & the ability to run completely silent on normal desktop use.
Under gaming loads all these psu's will still only run around 20dcb which will be quieter than ypur gpu & case fans.