I highly recommend Noctua D15
s - it fits perfectly! I made a mistake of originally ordering Noctua D15 (without s), and the video card could not be inserted into PCIe x16 slot, but only in the second slot (PCIe x8). Truth be told, you won't notice any difference in performance between the slots, however, you will be limiting yourself in the future if you decide to use a second graphics card. So go with Noctua D15
s. It's an awesome cooler - same as Noctua D15, but slightly different shape (asymmetrical).
PS. One important thing to note: Noctua D15
s - comes with only one fan. If you want the second one (comes by default with Noctua D15), get the
NF-A12x25 PWM (it's a 120mm version instead of the 140mm). If you get 140mm, the ram sticks will be in the way and the fan won't fit.
It depends on your future plans. If you plan to extend ram in several years to 64gb, then go with 2x16gb. If 32gb is all you'll ever need, then 4x8gb is perfectly fine. Other than the possibility of expansion, there's really not much difference.