What are your current parts now?
What is the main use for this pc?
There are some caveats:
1. One can not count on adding another 8gb of ram and expect it to work.
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
This is more difficult when more sticks are involved.
That is why ram vendors will NOT support ram that is not bought in one kit.
If you do buy more disparate sticks, they must be the same speed, voltage and cas numbers.
Even then your chances of working are less than 100%
What is your plan "B" if the new stick/s do not work?
Sometimes increasing the ram voltage in the bios will make things work.
If you want 16gb, my suggestion is to buy a 2 x 8gb kit that matches your current specs.
Then, try adding in your old 8gb,
If it works, good; you now have extra ram.
If not, sell the old ram or keep it as a spare.
2. If your main use is for multithreaded batch applications, then a i7 with more threads is very good.
If your main usage is for gaming, then faster single thread performance is all important.
Your motherboard does not support overclocking of a K suffix processor which is what you need to do for maximum performance.
3. What are you looking for to cause a change in motherboard?
If you are also looking at K processors, yes, you want a Z370 or Z390 based motherboard.
4. Intel ram controller is very good in anticipating ram need. Because of that Intel does not depend on fast ram.
Buying new, 3000 speed is about right, at most 3600 speed.
At equal speed lower latency is better.