monitor HP Omen X 25f and gpu advice?

[5hootemup]

I dont think your setup will get you >240 fps consistently. Your GPU is OK but your CPU is not. Look at i5-9400 if you are on a buget.

Hi my friend says the i5-9400 isn't compatible with my mobo?

[Max_hz]

Price is a bit high for a used product nevertheless just check the monitor for broken/stucked pixels, backlightbleed and IPS glow and other failures (someone send it back for a reason) , good thing you bought it from amazon so if any annoys you can always return it.
GPU is good for high fps rates in cs:go and other less demanding games
i5-9400 is socket 1151 so not compatible with your motherboard.
If you play with the graphic settings in game a cpu i7 3770 will reach between 200-300 fps in cs:go so for now fast enough you can always upgrade motherboard and cpu later.