Cheapest upgrade would be going with SSD as an OS drive if you already don't have SSD as an OS drive. Though, SSD won't give your more FPS in games nor does it reduce render time. But what SSD does over HDD is a lot shorter loading times; e.g PC boot up, game loading, applications loading, PC shut down. Video comparison between SSD and HDD,
youtube:
https://www.youtube.com/watch?v=j84eEjP-RL4
Best price to performance ratio SSD is Crucial MX500,
pcpp: https://pcpartpicker.com/products/compare/h3tQzy,4mkj4D,ft8j4D,nF8j4D/
(I too have MX500 in use with my Skylake build but mine is 1TB in size, full specs with pics in my sig.)
To get more FPS out of your games and also shortening your render times (provided you use your CPU to do the final render), new CPU-MoBo-RAM combo will give you that. E.g this combo:
PCPartPicker part list /
Price breakdown by merchant
CPU: AMD - Ryzen 5 2600 3.4GHz 6-Core Processor ($159.89 @ OutletPC)
Motherboard: MSI - B450M PRO-VDH Micro ATX AM4 Motherboard ($68.99 @ B&H)
Memory: G.Skill - Ripjaws V Series 16GB (2 x 8GB) DDR4-3200 Memory ($124.99 @ Newegg)
Total: $353.87
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-11-03 08:02 EDT-0400
i7-3770K vs R5 2600 comparison:
https://cpu.userbenchmark.com/Compare/Intel-Core-i7-3770K-vs-AMD-Ryzen-5-2600/1317vs3955
Since R5 2600 is 8 core 16 threaded CPU, it will give you far better CPU multi-core performance, reducing CPU render time quite a bit. Also, since R5 2600 has about 20% better quad-core performance than your 3rd gen Core i7, it can also give you a bit better performance in games (because most games use CPU's quad-core performance).
But if you want to have a bit better performance without spending a dime then OC your i7-3770K from stock 3.5 Ghz to e.g 4.0 Ghz or 4.2 Ghz.