Hi i have the Acer X34p (the 120hz variant of your monitor), I'm using 9700k and a RTX 2070 super, obviously it depends on the game but i generally get 100fps at max settings. The fps difference if you get will be 10-20 fps and if you think that's worth the cost go for it. Personally if i had a 1080 I'd just keep using it and although your x34 overclocks to 100hz i don't think you should buy a new card just yet.
The price of upgrading to a 2070/2080 super doesn't seem worth it.
To answer your other question I think your 4790k is enough for gaming and you won't see much of an improvement if any if you go grab yourself a Ryzen/new board as games aren't typically multi-core optimised anyway...
Again your system seems decent for most games and should last another year at 60fps+ (which although not ideal after being at 100fps - is playable), go on a holiday or something