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!