Best 'bang for buck' upgrade for final cut performance

Hi all

I have an older system that I looking to upgrade to increase performance in Final Cut 10.4. I am finding that I am unable to work/multitask when ever I start rendering a clip in the timeline. Both my CPU and RAM jump up to over 75% usage during the more intense periods.

Processor : 3.49 GHz Intel Core i7 Memory : 16GB 133 DDR3 GFX : Gigabyte GeForce GTX 650 1024MB Motherboard : Gigabyte z77x - ud5h Running High Sierra on a 512 SSD

I am unsure whether upgrading the RAM will increase work speed more than a new GFX card. But I am also unaware of what GFX card will work with my older motherboard. More than open to overclocking too. I have never done it before, but happy to experiment.

Thanks