I would buy a graphics card to replace the GTX960.
The Haswell i5 you have is reasonably good, and to get a meaningful upgrade you would need to spend $650 on a CoffeeLake 6-core ($300), a Z370 board ($125), a new cooler ($50+), some hideously-overpriced DDR4 ($200). After spending that $675 or more, you're still only going to get a 30-50% performance increase in CPU-limited games - and most games are GPU limited.
Now is a truly terrible time to buy a graphics card, but a GTX 1060 3GB isn't quite as desirable for the cryptocurrency miners that are (largely) responsible for all the price-hikes on graphics cards. You can find them starting from about $250 and they'll give about a 50% performance boost over the GTX 960. You could also look for a used GTX980 on ebay/craigslist etc, which would be a better performer for the same money but obviously comes without the warranty of a new graphics card.
I'm making the assumption that you game at 1920x1080 or lower, since your GTX960 wouldn't stand a chance at higher resolutions than that in modern games anyway. If you have a high-resolution screen, the next obvious upgrade is a 1070Ti and you will struggle to find one at a sensible price
If you don't already have one, an SSD for OS and games is highly recommended (the MX500 seems to be the best option for your money right now) and I would consider throwing another stick of DDR3 into your PC, since DDR3 is cheap at the moment.