Advice on getting the best for my system with limited funds?

[George O'Toole]

Operating System
Windows 10 Home 64-bit
CPU
Intel Core i7 4790K @ 4.00GHz
Haswell 22nm Technology
RAM
16.0GB Dual-Channel DDR3 @ 799MHz (11-11-11-28)
Motherboard
ASUSTeK COMPUTER INC. Z97-K (SOCKET 1150)
Graphics
G246HL (1920x1080@60Hz)
2460 (1920x1080@60Hz)
2460 (1920x1080@60Hz)
4095MB NVIDIA Zotac GeForce GTX 1080
Storage
447GB SanDisk Ultra II 480GB (SATA (SSD))
232GB Samsung SSD 850 EVO 250GB (SATA (SSD))
1863GB Western Digital WDC WD2003FZEX-00SRLA0 (SATA )
3726GB TOSHIBA HDWE140 (SATA )
465GB Samsung SSD 850 EVO 500GB (SATA (SSD))
This is the current specification of my PC. I am looking for suggestions regarding getting the best of what I have? Am I better to stick with what I have and maybe replace three monitors with 1 widescreen 4K monitor, or upgrade RAM or go the whole hog and replace motherboard, chip and increase RAM to 32GB?
This PC is only used for X-Plane! I am retired so have limited funding available?

 

[George O'Toole]

Thanks for the advice. I do think the bottleneck is the CPU rather than the CPU becuase the 1080 is only 8months old whereas CPU approximately 3 years...