You can find a rough guide to gpu performance at ton's gpu hierarchy guide:
https://www.tomshardware.com/reviews/gpu-hierarchy,4388.html
RX580 is rated at 57.9 and GTX1070 at 69.9
What is the make/model of your psu?
What was your graphics card that failed?
If it makes a difference, the rx580 needs a stronger psu.
While a FX-8350 has relatively slow cores, it still may be ok, depending on your games.
Try this test:
Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
Since you need a replacement card anyway, go ahead and buy one now.
I have no problem with used so long as you can trust the seller.
I think you get fair value from a used card.
Ebay is ok and has sufficient buyer protection if you pay with paypal.
Some EVGA cards may come with a transferable warranty.
If your games are fast action shooters, buy the strongest graphics card you can afford.
If your competitive games are multiplayer, look for a cpu with many threads.
Try removing one of your current 8 threads and see if it impacts your games.
You can do this in the windows msconfig boot advanced options option.
You will need to reboot for the change to take effect. Set the number of threads to less than you have.
This will tell you how sensitive your games are to the benefits of many threads.
If you see little difference, your game does not need all the threads you have.
If your games are sims, mmo or strategy games, pay attention first to the single thread performance.