The 3000 rpm Noctua fan is actually pretty noisy, I've got two S5's with it on them. Don't know why but it runs at 2700 rpm on one and 1800 rpm on the other at the same temps. Mine are the 140mm with adapters which move even more air.
I know they are noisy but from what I have seen it makes a noticeable difference with the stock fan, am I right?
No need for a second Noctua, one is enough for 325MHZ=1070Gh (certainly) and maybe even 337.5 (1100 GH) as long as ambient is below 25C.
Second Noctua in pull position does close to nothing.
It is ~56dB (at one foot distance), though, i don't know where manufacturers get the the bs number of 41dB
CX750M works fine, it's a workhorse. i had mine working with S1, S3 and now S5. No problem, but it WILL use ~5% more power at the wall in comparison with a gold PSU, but it is just ~$2.6 more per month at 12c/kwh.