Advice on Upgrading Graphics Card

Hi, I'm fairly new to gaming computer stuff, and I'm looking to upgrade my graphics card (It's a pre-built PC.) It would be a budget card, as the highest I'm willing to go for now is $300. And I'm in the United States.

I currently have an AMD Radeon RX 560. The CPU is an AMD FX-6300, if that's important to know.

For an idea of what I'm looking for, I just recently got Rage 2 and can't really play it, so I need to upgrade for that. I'm also looking at Borderlands 3 once it comes out, but I'd like to hold onto the card for a while before having to upgrade again.

I don't really need super high graphics or anything, and not expecting it with my budget. Honestly, playing at medium graphics is fine for me... Anyway, as I said, I'm fairly new, but I've been trying to look at options on my own.

I saw an AMD Radeon RX 580 on sale with Amazon, and according to some benchmarks I found online, it does seem a lot better (I also saw in recent posts that it was suggested, so is it a decent upgrade?) Other than that, there are so many that I'm not really sure what would be best. Help would be much appreciated.