It depends on your budget and what your current hardware is.
This link is the most helpful :
Another link to a useful video:
Always prefer Nvidia to AMD to choose your GPU. This will be the key element.
For example, my config :
Dedicated 480 GbSSD but my orthophoto scenery is on a 2Tb HD
1 FHD monitor
I'm quite happy about it (30 to 40 fps most of the time) because I bought it 3 years ago and I don't intend to change it now. But if I decided to upgrade it, I would go for a GTX1070 or even 1080.
Guillaume (not affiliated with Laminar Research)