1) The motherboard isn't designed to overclock a "K" series if that matters, but AFAIK the i7-4790K or i5-4670K are pretty expensive even used. I looked quickly and I think a used i5-4670K starts at $160 and i think it's roughly 15% faster without overclocking (assuming you stick with 3.9GHz vs 3.4GHz locked or similar thus 39/34 thus maybe at most 15% or so higher FPS in some games).
*speaking of which, it's a small difference to default but it MAY be possible to go into the BIOS and set all cores (via the Multiplier values likely) to the max which I believe is "x34" (so under load it doesn't drop below 3.4GHz which is only 6% faster than 3.2GHz... hey, if it ends up free performance why not?).
But as for the i5/i7 upgrade again it's hard to recommend to keep putting money into the system. I'd wait then eventually replace with either a new AMD AM4 rig (i.e. R7-3700 8-core in 2019) or a similar value Intel setup.
2) again, on a per-game basis look at GPU Usage/frequency (VSYNC OFF) to estimate gains with a better CPU
3) Which monitor?
I looked at PCPARTPICKER to guess which one you're looking at. To be clear, you didn't buy yet?
THIS one is 27", TN, 2560x1440, 144Hz (with Freesync though that only works with AMD GPU):
https://pcpartpicker.com/product/t4Crxr/acer-monitor-umhg0aa001
(my main issue is it's TN. I'd get IPS for better color and less fading off-center... no GSYNC either but IPS with GSYNC is $700 or so)
I personally would NOT get a 1920x1080 monitor since many games (especially ones with small HUD/text like CIV5) look better as does the DESKTOP obviously with sharper text (I use 125% DPI scaling though).
*The monitor I like best that isn't ridiculously over budget is THIS one for about $500USD:
https://pcpartpicker.com/product/sqp323/acer-monitor-xf270hu
27", IPS, 2560x1440, 144Hz (and Freesync)