what if I already have a dedicated GPU? (RX 580 8gb) I've heard 9400f is a lot more powerful than even R5 2600 which is more powerful than 3300x. also, I don't think I'm going for an upgrade anytime soon.
The 9400f is not even close to being 'a lot more powerful'...only 1 or 2%
in this review at 1080p. And they're using a 2080ti, a huge mismatch no sane person would consider for their rig, that skews the comparison a lot. That's essentially no difference at all and a less powerful GPU (like your RX580) won't squeeze anything close to that.
In any other tasks, like just running the OS, web browsing, office productivity, they trade blows with the 9400F being favored by known Intel optimized apps and the 3300X favored by apps that like threads. The 2600 is really only going to excel at apps that like a LOT of threads, and cores.
The 2600's older arch is less efficient but it's got 6 cores/12 threads going for it against the 4/8 for 3300x and 6/6 for the 9400F. But when considering the 2600 you can also count on some pretty good overclocking gains but not so much with the 3300X and none with the clock-locked 9400F. If you do anything at all alongside gaming: holding Facebook open in Chrome, streaming your gaming, etc. then you'd probably find 2600 a much better choice simply for all the cores and threads. But overclocked (on a good B450 motherboard I wouldn't be surprised to see 4.0-4.1Ghz fixed, all core) it would be a beast...and a decent pairing even for that 2080ti.