Do you have a budget?
For gaming, the graphics card is all important.
The $300 GTX1660ti represents a significant boost in that category.
One rule of thumb for a balanced gamer is to budget 1/2 the cost of the graphics card for the cpu. You are about right in that category
The i5-10400 and the 3600 each have 12 threads which is sufficient.
only 4-6 of them are likely to be effectively used for a game, leaving plenty left for streaming.
The single thread performance is similar.
The performance of the single master thread is what games need most.
Functionally, you would be equally happy with either processor.
On ram, intel is not tied to ram for performance.
2666 speed is fine. But ryzen does, look more to 3600 speed for the 3600.
On drives, I suggest abandoning the HDD. You can always add storage later .
On a budget, look at the 1/2tb intel 660P or 665P.
The top reasonable performer is going to be the samsung 970 evo plus.
I do not think the 980 pro is worth it if you are budget constrained.
I like intel and samsung because of quality.
They make their own components and can do a better job of integration.
The drive is one place where you can't tolerate issues.
If you have aspirations of upgrading a i5-10400 to a stronger K suffix processor or to the upcoming rocket lake, then a Z490 based motherboard is the correct pick.
If that is unlikely, then any lga1200 motherboard at half the price will do.
Similarly, if you might upgrade to a 5000 series ryzen, spend a bit more for a X570 based motherboard.
ryzen 5000 looks very good, primarily because of the 19%ipc gain.
Rocket lake is rumored to be similar, but that will not come until Q1.
Cases are a personal thing.
Buy a case with the looks you love.
I recently came across a review of the Lian li lancool mesh:
https://www.tomshardware.com/reviews/lian-li-lancool-ii-mesh-performance-review
www.newegg.com
There is a RGB version also.
Your psu pick is reasonable.
I would think 750w would be fine, even for a 3080 future card.
I have no problem overprovisioning a PSU a bit. Say 20%.
It will allow for a stronger future graphics card upgrade.
It will run cooler, quieter, and more efficiently in the middle third of it's range.
A PSU will only use the wattage demanded of it, regardless of it's max capability.
I like the seasonic focus units. This time of year, Seasonic often has sales; look for them.
Here is a good 850w unit with a 7 year warranty:
https://www.newegg.com/seasonic-focus-gm-series-focus-gm-850-750w/p/N82E16817151231
On your future dream builds, look for ryzen 5000 series processor or intel rocket lake. The better ipc is what will distinguish them.
3080 is currently the card to get if you will game at 4k resolution.
Put in your plans a large wide high resolution monitor.
It will be more immersive for gaming.
Multi window capability is great for normal desktop work.