Current Build - Possible Upgrades?

My computer specs at the moment:
CPU: i7-6700k (not overclocked yet)
CPU Cooler: Corsair H110i
GPU: AMD Radeon R9 390x
Motherboard: Gigabyte GA-Z170X-Gaming 7
RAM: 16GB Corsair 2133MHz (2x8GB DDR4)
Storage: 2TB HDD (bog standard think it might be a hybrid)
Storage2: Samsung SSD 850 EVO 250GB
Fans: Noctua NF-A14 industrial PPC (4 of them excluding the 2 Corsair fans on my cooler which has the same RPM as the Noctua)
Power Supply: XFX TS 650watt 80 plus bronze certified
Case: NZXT S340 (I think?)
Monitor: BenQ XL2430T 24inch
Mouse: SteelSeries Sensei RAW (basic cheap mouse to be honest) with a SteelSeries mouse pad.
Keyboard: Corsair K70 with Cherry MX Brown Keys
Headset: Astro A50's (none wireless)

So I was thinking about possibly getting more RAM although it's quite expensive. I was also thinking about getting a second monitor as one is good but would be so much nicer having two, maybe if I get a higher quality monitor maybe 4k but idk, also widescreen does look nice but I don't have much space on my desk.

Any thoughts and suggests are welcome, Thanks!

more ram wont really help, the only thing i would see that gives you a real improvement is either a 1080(ti) and a second monitor, also you can overclock the cpu especially with the aio you have

a choice for what you need depends on what you will be going to be doing with it

do you want to game or do work, or maybe even both?

Damn, your build sounds sick! Enjoy it!

if you were to work a lot on it then id recommend getting a 4k one since you can just fit more stuff on it without losing any space, and if you were to play then id recommend getting one with freesync and also one with an ips panel cause color quality is a lot better than on the tn panel, i got the same one and am looking for a nice ips monitor which i can use as primary one