Hi, Intel will change socket, they already said they will.
AMD's AM4 socket still have one more Ryzen Gen up ahead, Ryzen 4xxx, to be launched next year.
I have to agree with Darkbreeze here, I believe too that right now, the Ryzen 7 3700X is basically one of the best all rounded CPUs, it comes with a very decent and good stock cooler, have a great price point, you can get a cheap mobo and still overclock (if thats what you like to do), and have the same amount of threads as the pricier Core i9 9900K (which doesn't include a cooler).
Keep in mind that depending on the game, while Intel may still have the lead on most, AMD now have very powerful new beasts out there and as soon as you go up the resolution (from 1080p to 1440p or 4K) the games become more GPU bound and so the FPS diference become really low no matter what CPU you choose of those you mention.
Thats why its is impotant to know whats your playing resolution. If its 1080P, then maybe the Core i9 9900K could make a diference, but as soon as you step up to the 1440p landscape any of those CPU will do great.
For the memory, if you can sell yor current cpu+mobo+memory togheter, then do it, and get a good 2x8GB DDR4 3200MHz low latency kit (CL14) or a good 3600MHz one. Memory prices are at a really sweet spot right now, so its a great time to grap the best you can if you live in the US!
Cheers
PD, if money is not a problem, avoid the 9700K <- its not a bad CPU for gaming, in fact is really good one, but Intel disable the HT on that one, so its only an 8 cores / 8 threads cpu, and if you're streaming and maybe doing some rendering, you will want the extra threads from the i9-9900K or the R7-3700X/3900X, etc.