I choose 64GB of RAM for 2 reasons, one is as you said memory is cheap and it allows it to be used as shared VRAM. Not sure how helpful it could be, didn't see any benchmarks on it.
A descrete video card like the RTX does not share/pool video ram with the PC's main system ram.
On a side note, I would not reccomend using the same space as your xbox. ff it is in an enclosed area, the 14900k will get sigificantlly hotter and airflow is much more important to a PC. You can look at watercooling AIOs for your CPU, but it still needs significant airflow for intake and exhaust.
I would highly suggest talking to a a reputible shop like Microcenter or local shop that can provide you with the right guidence for you needs. I was also wrong about the 14600K, there is more of a difference in frame rate on some games than the 14900K. The 14700K usually tests within 2-5fps of the 14900k if you want to save some money.
Also be aware that there is currently an issue with the intel 14900k / 14700k having a 9% decrease in performance due to motherboard manufactuer settings being to agressive.
If I were building it, I would suggest a 14700k, 32GB RAM and then spend the savings on the video card.