Buying a 3440×1440 144Hz Graphics Card – Desktop PCs: Buying Tips

You have a previous thread regarding my problems and got a lot of good responses so I will continue to ask for help here.

My old computer is starting to get messy so I go and do online troubleshooting.
Most of the answers say that it is probably my motherboard that is giving a little bit of power to my memories, which are not working as they should be. (it does not load drivers and gives random memory errors)
Just to be safe, check your graphics card and processor at the web hall as they have a troubleshooting service.
And the result is that they work?!
Well, this motherboard is broken.
Since they are too old to buy something similar, I am collecting and scraping them together for a new motherboard, memories, cooling and processor.
Get all the new parts and put the computer back together and the same problem will come back?!

The only thing I have left is the PSU and graphics card.
So the graphics card was the problem (which was fine according to our troubleshooting)..
So..
Now I need some tips and findings regarding graphics cards, which for me is one hell of a jungle!
I had a good ASUS ROG 1070 GTX that I found very cheap on Black Friday.
But now.. what the hell should I replace it with?
Sitting on a 3440 x 1440 – 34″ – 144Hz widescreen monitor which the new card should be able to handle.
Ideas and tips?

I aim to be able to play Bethesda Starfield without issues this fall when it’s released.