Don't pair anything other than Z390 motherboards with a 9900K. It consumes more power than even the R9 3900X, heck it consumes almost the same power as the TR 2920X. Pairing it with mid-range boards like the Z370 or the B365 will bake the VRMs as they're often 4 or 6-phase VRMs. I recommend the Z390 from Gigabyte, as they're using 12 phase VRMs on their Z390 boards.
As for the memory, 3000 MHz is totally fine for an Intel gaming system. The GPU, however, I'm afraid will have space issues as you are using the H150i. The Masterbox MB520 isn't exactly a big case, and will likely not support 280mm + 140mm radiators at the same time. If you want to do it, you can look at cases that are bigger, for example the Bequiet Dark Base 700.
The PSU that you have already must be a good unit. Your system might consume north of 600W if overclocked. For the monitor, I'm not too good at it, so I won't comment on it.
So Here's my pick:
PCPartPicker Part List:
https://pcpartpicker.com/list/vfKHfH
CPU: Intel Core i9-9900K 3.6 GHz 8-Core Processor ($493.99 @ SuperBiiz)
CPU Cooler: Corsair H150i PRO 47.3 CFM Liquid CPU Cooler ($159.99 @ Newegg)
Motherboard: Gigabyte Z390 AORUS PRO WIFI ATX LGA1151 Motherboard ($189.99 @ Amazon)
Memory: G.Skill Trident Z RGB 32 GB (2 x 16 GB) DDR4-3000 Memory ($149.99 @ Newegg)
Video Card: Gigabyte GeForce RTX 2080 Ti 11 GB GAMING OC Video Card ($1129.99 @ SuperBiiz)
Case: Cooler Master MasterBox MB520 RGB ATX Mid Tower Case ($89.99 @ Amazon)
Monitor: Acer Predator Z1 31.5" 2560x1440 165 Hz Monitor ($529.99 @ Amazon)
Total: $2743.93
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2019-09-04 03:47 EDT-0400