Hi, starting from these specs, I would personnaly go for GTX1070, or even GTX1080 from Nvidia. X-plane currently takes more advantage of Nvidia GPUs than AMD GPUs.
If needed, you could also add 8Gb of RAM.
That would already be a good start.
But if I were you, I would wait half a year (or maybe a year) until Laminar Research realeases the upgrades of X-plane 11 that will take advantage of Vulkan technology. More information on this page :
Just type Vulkan in the search bar.