Decent arma 3 build?

Solution
A bit over budget, but this will last you much longer. You can also get the RX480 to ramp down the cost. Not much difference in performance.

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i7-7700K 4.2GHz Quad-Core Processor ($343.49 @ OutletPC)
CPU Cooler: CRYORIG H7 49.0 CFM CPU Cooler ($34.89 @ OutletPC)
Motherboard: Gigabyte GA-Z270M-D3H Micro ATX LGA1151 Motherboard ($119.89 @ OutletPC)
Memory: G.Skill Ripjaws V Series 16GB (2 x 8GB) DDR4-3200 Memory ($119.99 @ Newegg)
Storage: Sandisk SSD PLUS 240GB 2.5" Solid State Drive ($78.99 @ Amazon)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal...
A bit over budget, but this will last you much longer. You can also get the RX480 to ramp down the cost. Not much difference in performance.

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i7-7700K 4.2GHz Quad-Core Processor ($343.49 @ OutletPC)
CPU Cooler: CRYORIG H7 49.0 CFM CPU Cooler ($34.89 @ OutletPC)
Motherboard: Gigabyte GA-Z270M-D3H Micro ATX LGA1151 Motherboard ($119.89 @ OutletPC)
Memory: G.Skill Ripjaws V Series 16GB (2 x 8GB) DDR4-3200 Memory ($119.99 @ Newegg)
Storage: Sandisk SSD PLUS 240GB 2.5" Solid State Drive ($78.99 @ Amazon)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($49.33 @ OutletPC)
Video Card: EVGA GeForce GTX 1060 6GB 6GB SC GAMING Video Card ($239.99 @ Amazon)
Case: Deepcool TESSERACT SW ATX Mid Tower Case ($38.99 @ SuperBiiz)
Power Supply: SeaSonic S12II 520W 80+ Bronze Certified ATX Power Supply ($47.89 @ B&H)
Operating System: Microsoft Windows 10 Home OEM 64-bit ($88.58 @ OutletPC)
Monitor: AOC E2425SWD 24.0" 1920x1080 60Hz Monitor ($99.98 @ Jet)
Total: $1262.01
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-03-27 19:33 EDT-0400
 
Solution
I suggest a better GPU and a non-k i7, despite Arma 3 being very CPU heavy, this will net you better performance and allow you to squeeze in a 1440p 60Hz monitor.
PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i7-7700 3.6GHz Quad-Core Processor ($288.49 @ SuperBiiz)
CPU Cooler: CRYORIG M9i 48.4 CFM CPU Cooler ($19.88 @ OutletPC)
Motherboard: Gigabyte GA-B250M-DS3H Micro ATX LGA1151 Motherboard ($68.49 @ SuperBiiz)
Memory: G.Skill Ripjaws V Series 16GB (2 x 8GB) DDR4-2666 Memory ($99.97 @ Jet)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($49.33 @ OutletPC)
Video Card: EVGA GeForce GTX 1070 8GB SC GAMING ACX 3.0 Black Edition Video Card ($374.00 @ Amazon)
Case: NZXT S340 (Black) ATX Mid Tower Case ($66.99 @ SuperBiiz)
Power Supply: SeaSonic S12II 620W 80+ Bronze Certified ATX Power Supply ($49.90 @ Amazon)
Monitor: Acer G257HU smidpx 25.0" 2560x1440 60Hz Monitor ($209.99 @ Newegg)
Total: $1227.04
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-03-27 19:55 EDT-0400