2 questions based on your feedback:
1) Should I hold off on upgrading my CPU then, until I buy a GPU to upgrade from the R9 390?
The Ryzen 7 5700X3D for $145 seems like a pretty good deal at the moment but I can wait if there won't be much point of doing it now.
I don't know your actual budget. Since you're on AM4, 5700x3D makes perfect sense. It doesn't seem to be something that'll be beaten in price/perf, nor shall it go much lower. $145 for a low bin 5800x3D is a steal frankly, the high bin one is a freaking 365!
(RANT)
I'm starting to think we should be making social media hate accounts on the 5800x3D, it got so much praise even though a mere 7700 basically wrecks it, and it's so expensive for what it is...
The more modern (and still top of the line) 7800x3D is 450$, that is significantly more power efficiency, performance, better V$ implem, in every way it's 20-30% better.
LOL now that I check, the lower bin 7700 is more expensive than the 7700X. That hit AMD did with the "let's boost them to insanity for 5% perf to NOT beat Intel and then let's release the sane ones on a lower bin" sure worked great against themselves. Much like the 65W 9700X...
2) Should I only aim for a GPU with 12 GB minimum? I think the lowest price option from the AMD perspective is a 6700?
Er that one is extremely questionable.
The rule of thumb is:
8Go for 1080p is a little tight
12Go for 1440p is comfortable
16Go for 4K is also a bit tight but better than 8 for 1080p yet
If you don't intend to play at 1440p in the lifetime of your card, it's a bit unnecessary, but not bad. Plus I don't know, you might want to do extensive video creation at some point (basic stuff runs fine on 8Go).
If you do intend to jump to 1440p, you're better off future proofing, but personally I'd still favour something like AV1 encode/decode over an extra 4Go you're not even sure of using. Most people think "I dun care I don't stream or record", but IMO it's the kind of thing that when you find out that you're into it, you can't stop. And AMD has got really poor h264 encoders even on RDNA 3 (it's basically around Pascal/Turing level) but solid h265 and AV1. If you're gonna record h265 is fine enough, but for streaming AV1 will be everywhere in a few years time. Honestly if it weren't for Twitch being an empire of h264 spaghetti code, it should already be.
Heck a 7600 has the same price/perf ratio than a 6750XT on Amazon right now. 20% more cost for 19% more perf on the 6750xt.
Although honestly, if AMD has already announced a OCT 10 event to "present something" that's definitely RDNA 4, I'd just wait for that and see what prices they're going for. Even just N44/8500XT promises to be a solid buy.