Congrats on the free RTX 3090!!
As far as what your asking for, you unfortunately cannot upgrade to a 12th gen processor without upgrading your motherboard and possibly ram depending on what motherboard you choose. This maybe easier to do since you are planning to upgrade your case anyway, since you would be pretty much rebuilding your unit.
You can upgrade to an 11th gen i9 without having to switch out your other components.
Let me know if this helps!
It always depends on what software is running, whether the CPU or GPU is used more or less than the other. There's really no such thing as a perfectly matched pair that run exactly the same. An 11th gen i7 should be quite reasonable with a 3090.
3090 release date: Sept 2020
i7 11700 release date: March 2021
So you see, they are both very close
What exactly is this MSI case in question? What did it come with in terms of fans and what is the CPU cooler? Have you been monitoring the temperatures of the CPU and GPU to see if they are getting to their thermal limits? Crappy cooling is your enemy not this CPU/GPU combo.
11700K throttle temp is 100C, below that may seem hot but it is not affecting performance.
3090 throttle temp is 93C.
Whatever is affecting your system performance, it is software or driver or something like that. Not because anything is overheating to the point of throttling. Also not because either the CPU or GPU can't keep up with the other.
Sorry, I have a Ryzen/Radeon system and can't help with the details of Intel / nVidia drivers and software.
PS - The correct term for the CPU cooler is to call it a "240 AIO"
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