Spend more to get a Z270 ITX serverboard or recrtified, there is very little performance gain for switching to a 9700K or 3700X.
Will be spending $250-$300 on it, but better than $500 for new CPU + MB for almost zero performance gain - throw the difference into a better GPU.
Pretty loaded question @Polski03 . How far are you downsizing? Im my particular case, I like the SM560 a lot. If you are going to use something that size, to me, it makes a heck of a lot of sense to stick to one of the 65 watt tdp parts like the 3600 or 3700x. They are simply much easier to cool than the higher watt tdp counterparts.
Certain games like Far Cry 5 start to show some limitations in a 6 core so I decided to go back to a 8 core cpu in the 3700x. I had a 2700x but found it too hard to keep cool with the small coolers like the Noctura L9a. I downgraded to a 2600 and it was miles easier to keep cool and stays quiet. Quiet is my main factor anymore in hardware.
You have to determine exactly what you want out of it. I could easily upgrade to a SM580 case and go all out, I simply don't need to with the 3700x.
@KSliger is right. To me however, I rather spend some more to get a lot of updated tech and a cool running cpu, rather than dumping that kind of money on a old system. Its older and still good, but there won't be any room to move up again in the future. z270 is a dead end chipset.
I would look around for some deals on ebay. I'm sure lots of ppl getting rid of 2x Ryzen processors, and even new ones were on hella good sales.
On a different note, I hope everyone who celebrates it had a great 4th.
Better GPU isnt needed to me as I already have a 2080ti and I ordered the SM580. Also I'm not a fan of buying used and couldnt find any refurb z270s besides with the pricing of those used boards it almost equaled to a new i5 or ryzen 5 with a new mobo.