What upgrade next!

[xraijin]

Hello everyone,
I have an outdated PC (since 2014) and with a limited budget im trying to upgrade it one piece at a time.
My outdated PC Specs:

• Operating System: Windows 8.1.
• Motherboard: MSI z97 Gaming3.
• PSU: ZALMAN500-HP (500W).
• Processor: Intel Core i5-4460 @ 3.2 GHz up to 3.4(never OC it tbh).
• Memory: 8 GB RAM.
• GPU: GTX 750Ti Twin Frozr.
• Hard Drive: 120GB (SSD) + 2TB (HDD)

The Changes that i made so far:

• Memory: 8 GB RAM >>> 16 GB RAM (i know that those should change when i change my MB since they are DDR3 but 8GB was low for my new GPU so i had to add an other 8GB)
• GPU: GTX 750Ti Twin Frozr >>> PNY GTX 1080 Ti xlr8 OC 11GB.
• PSU: ZALMAN500-HP (500W) >>> CoolerMaster 750W. 

Next I'm aiming for CPU+Motherboard, I think im going for i9-9900k cuz as i saw from many youtube reviews that i9-9900K gives the best possible performance with my GPU.
So my Question is if its true that i9-9900k is the best combo for my GPU & what Motherboard i should go for with that combo for the maximum performance,

If not what's the best CPU that i should aim for & what Motherboard would be the best to give the Max performance with that CPU and my GPU.

Last but not least, should i go for AirCooling or WaterCooling for my CPU, and should i do Air/Water Cooling for my GPU or the 3x integrated fans that comes with my PNY GPU are enough?.
Thanks in advance guys.

[wall03]

what is your budget? If you are going for something like 9900K and 1080 Ti I guess your budget is ~$1000 total (please don't get Intel). Ryzen has value and IDK how much the 9900K costs but get a Ryzen.

[boggy77]

a ryzen 7 3700x has 90% of the performance of the 9900k, while being cheaper and more easy to upgrade. it's also cooler and consumes less power.

in terms of air vs liquid, i'm a fan of air coolers. for a 3700x, you don't really need a big liquid cooler as it doesn't draw that much power anyway.

[xg32]

if you have a high refresh rate 1080p monitor, a 9900k isn't bad, but still not good value, for 1440p/4k, r5 3600 would be the best value, 3700x if you have cash to spare.

[dizmo]

The 9900k makes no sense now that the 109700k is almost out.