Your CPU may be a bottleneck with anything beyond a GTX 1060. Overall the 970 is still a decent card which can offer 60fps in many games at 1080p with the details turned down a tad.
First the basics:
Have you performed any maintenance on the PC?
Opening the case and cleaning out the dust with a can of compressed air.
Performing a clean-up of the system using software like CCleaner. In severe cases backing up your data and performing a fresh install if the system has become bogged down over the years.
Ensuring all drivers are up to date.
As for upgrades the following would help:
SSD Drive - Doesn't boost frame rates but will make your PC much more responsive and quicker in day to day usage.
GeForce 1660Ti - Faster than the 970 and since your Power Supply supports the 970 there should be no issue with such an upgrade. It will be slightly bottlenecked by your CPU however.
Doubling your memory, 16GB is really the sweet spot for a gaming PC nowadays.
It could also be possible to upgrade to an i7 4770, however with what you'd pay for a new processor you might as well in this case get a current entry level CPU, motherboard (current one wouldn't work) and RAM (newer systems use DDR4).
These suggestions would help your system, my personal recommendation if you're going to upgrade the hardware, is to carry on using what you have currently. While setting aside some money for a new build. 4 years is a pretty good run for a gaming PC.
A comparison of the 1660Ti versus the 970 with many popular games can be found here.
Good luck with whatever you decide.