If you still dont find it quiet enough for your liking, look into Prolimatech MK-26 and attach a pair of quiet 120mm fans, run them at 500rpms or so, and should be inaudible.
And what about the AC Accelero Extreme IV?
Well while i do think its a great cooler, it has a some drawbacks, with the back mounting plate that i like..... it becomes also a 4 slot cooler, kinda like MK-26 sort of speak, the MK-26 is a little better than accelero, specially under low rpms, SPCR Prolimatech MK-26 Multi-VGA Cooler
, but its strength imo is that you can use whatever fans you like, specially someone looking for something extremely quiet, you can throw in some 500-800rpms 120mm fans and it should be inaudible and still sustain the heat of a high end gpu. Im still very happy with accelero, mostly i wanted to be able to control the fans, but at the end there isn't much ramping as the cooler can handle a gtx780 very easily, even after 5 hours of playing i never seen it pass 65C, and the fans never increased more than 150rpms, so its very similar at the end than running it with a steady voltage like I was doing on the Accelero III, so with this in mind and both are 4 slots now.... i would prefer the mk-26 and a pair for scythe gentle typhoons 500rpm or 800rpm version should run fine at 12V. But this is my last persue of aftermarket gpu coolers, i really wanted to do it, now that its done, when i move on to a newer gpu, ill probably will stick with MSI twin Forzrs or whatever is more quiet at the time, nvidia gpus, at least this gen, are not that hard to cool like GTX480 was, although AMD does seem to have a very hot running gpu with 290X, but i still think its no where near how it was on the past.
Right now they do not have the noctune 14S in stock, just the regular 14 and 15, is there any difference?
If you are talking about the NH-D14 and NH-D15, i would go with the NH-D15, as quest posted, it comes with with 4pin pwm fans, while the D14 most of the time comes with 3pin fans. Most motherboards have its CPU_FAN header as a true PWM fan header, so it should be easier to control.
I think i might place the computer in my pantry and run 5 meter hdmis again, but i still need a good cpu cooler i guess.
My GamiMi setup (check sig) will be very similar to what you want to build, my case is located like 2mts from me, i cant hear it at idle, and even gaming i never notice it, where i do is under stress tests but this are scenarios that i never reach under usual day to day basis.
I took a 750 watt psu because of future sli support. Should be enough right?
I think it should be enough, but worth mentioning that the RM 450/550/650 are 160mm, once you go into bigger models the are larger, so might prevent the use of the bottom 140mm fan in the future if you decide to add this slot. I went with the biggest PSU that still remain on 160mm which is Seasonic SS-860XP2, their 760W version should be the same length in case you are interested, but some people have found coil whining and electric noise on them, i own two and mine are perfect no issues to report on either.
Since you are going with Asus Z97, should run FanXpert2 (or 3), so once you finish assembly and formatting, install Asus AI Suite III, and connect the Fractal 140 fans to CHA_FAN headers (it should have at least 3), and connect the NH-D15 fans (NF-A15 PWM) one to CPU_FAN and the other to the CPU_OPT, then run FanXpert tunning to stablish the max and mins of all the fans, check the suggested fan curves, and decided if you want to change them for a more quiet or cooler setup, etc. After this decide if you want after market fans or more fans, etc.