I have a Ryzen 5 2600 paired with a
GTX 1070 on an MSI x470 board with
16 GB DDR4 2666 Mhz RAM.
I WANT to upgrade to a Ryzen 7 3700X, but is it practical?
Is 4 more cores and a 200 MHz increase in base clock worth $329?
As the saying goes, if it ain't broke, on't fix it. If you're happy with the existing system and you're not a heavy multitasker, then I'd say you're good as is. Also, if you're not into professional content creation such as rendering, you're not needing that 3000 series processor.
FYI, you should ideally be on a DDR4-3200MHz ram kit to get the best out of your platform.
With that GPU and what games you can play at high settings, 2600 should be just fine. If you are doing some serious work, than 3700x would do much better. There's also 3600(non X) with practically same gaming performance as 3700x but at much lower price and could give you up to 30% or more performance boost in some games over 2600.
Your best upgrade currently would be getting a 2x8GB DDR4-3200MHz and selling your old one. Then, either a Ryzen 3600 or RTX 2070/better. Games don't typically utilise 8 cores at the moment. You'd get the same or better performance on a 3600 than you would on a 3700x. However, I don't think from 2600 to 3600 is worth it, so maybe wait until Ryzen 4600 before a CPU update.