i9-9900 is validated in 5680.  It has been done (see user benchmark).  this means you can also use i9-9900k if you have a good cooler for it. for gaming it depends on your gpu and the resolution you play at. for example, if you have 3060 Ti, seems improvement is very slight.   vs
I want to upgrade my CPU in my Inspiron 5680 i7-8700 @ 3.20GHz 6 cores to an i9 CPU. I need some tech support on the right CPU. Has anyone done this and is it worth it? Motherboard is a Coffee Lake. HELP!!!
cpu upgrade inspirion 5680 i7 to a i9