Hi Checkride,
Have a look at the recommended specifications for X-plane which can be found here http://www.x-plane.com/kb/x-plane-11-system-requirements/ .
Before you purchase your new computer download the demo version and try it. It can be found at http://www.x-plane.com/desktop/try-it/. I would suggest you go for the latest video card. If you want good performance use nothing other than NVidia.
I would also go for a power supply with a minimum of 650 watts. Why? You need a decent powere supply to cope with the power consumption of your new graphics card and have some reserve to cope with your other add-ons.
What do you want to use the computer for? Just for X-plane or as a general purpose home/office computer and run X-Plane as an auxilary?
Have a look at this link at
https://www.youtube.com/watch?v=j_YQMSeNM-M.
Hope this helps.
Glenn