What power supply for my system?

UNKNOWN5

Prominent
Apr 5, 2017
20
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520
Hi all,
I haven't bought a power supply before and I need some help.
I have the Phanteks P400s Tempered Glass Mid Tower Case which has a covered shroud so the psu wont be on display.
I went on cooler masters website and filled in my specs to see what wattage psu I need and it returned 500W, but as I want to future proof my purchase and give me the chance to get another gpu and run them in sli in the near future I want to make sure I have good headroom.
I am considering going 850W but I hear the farther you are from using the maximum, the less efficient the psu.
It would be nice if it was modular to reduce excess cables but the cost gaps seem quite large from semi-modular to fully modular, however, I do understand the quality improvements with the more expensive options.
I am thinking of buying this Cooler Master GM Series G750M PSU but is it worth buying a higher quality, more efficient psu? like the Corsair RM750x 750 W 80 Plus Gold which boasts a fully modular design with more efficiency and better cooling, the price is nearly double tho...
The more expensive option also has an 850W version for only around £10 more, for so little it seems I might as well go for the extra capacity to make sure I never run out.
Does anyone have any thoughts on these options or know anything I don't that could be of help?
Many thanks guys
 
Thanks for ur reply,
Those psu's do look very nice and have a lot of good sounding tech but do u know of any problems with the cheap coolermaster psu I listed?
just curios on the differences between the price points, is it worth paying nearly double for a top quality one?
Thanks again