What should I upgrade on my pc for under 600 USD

[DigitalDavid]

Current parts:
- Asus Z97-AR
- Samsung E 500GB 850 EVO
- Intel core i7 4790K
- EVGA GTX960 SC 2GB
- WD 1TB blue hd
- Crucial 16GB(8x2) D3 1600
- Corsair RM750i
-Samsung S27D360H 27.0" 1920x1080 60Hz 5ms

What should I upgrade? I mainly use my computer for gaming

 

[lucas_7_94]

Its a solid build. I would upgrade the graphics card if you're playing @ 1080p.

 

[theyeti87]

What is your monitor's resolution and refresh rate?

The GTX960 stands out to me as candidate for upgrade.

 

[WildCard999]

A GTX 1080 would be a good upgrade for the price.

 

[Zerk2012]

Current parts:
- Asus Z97-AR
- Samsung E 500GB 850 EVO
- Intel core i7 4790K
- EVGA GTX960 SC 2GB
- WD 1TB blue hd
- Crucial 16GB(8x2) D3 1600
- Corsair RM750i
-Samsung S27D360H 27.0" 1920x1080 60Hz 5ms

What should I upgrade? I mainly use my computer for gaming

A GTX 1070 will max out everything with your monitor resolution and refresh rate.
Might add a Good CPU cooler if you don't already have one and overclock that up a bit.
Other than that their nothing worth upgrading to from what you have.