I'd not be putting any more money into old systems for a couple more hundred MHz...
Wait till summer or fall, as this summer (rumoured July?) brings AMD's 3000 series desktop CPUs, where hopefully you can acquire a nice 6-8 core CPU that is FAR more efficient and better performing even if near the same clock speeds. (Rumours of AMD's 8 core offerings equaling or surpassing the 9900K's performance are very interesting!)
Goofy Intel is bringing out a 10 core 14 nm CPU this fall (allegedly to seem to keep with/counter 12 core Ryzens?), but, I can't imagine this even keeping up with 9900K in most games due to thermal limitations and lower clocks needed for TDP limits. (IMO, we want a die shrink so we can have more clockspeed in 6-8 cores at lower power, not two more cores and 25% greater heat within the same surface area heat spreader...)