I do not have the answer since this is a very complicated and situationally dependent question...but
Here is what I have
Asrock x99 Extreme4 mobo
16GB DDR4 2400 Ram
SSD hard drive (xplane installed there too)
EVGA GTX 960 SC ACX+ 2.0 4GB video card
I have SkyMaxx Pro v3, UrbanMaxx v2 3D and Maxx FX plugins loaded and fairly high settings
I have X-Plane 10 latest stable release
I have my graphics set pretty close to Medium
I get about 24 fps...which is not really acceptable.
I plan on upgrading my video card to an EVGA GTX 980 ti SC ACX+ 2.0 6GB this is both to allow more fps in X-Plane as well as get me ready for the Occulus Rift VR head gear in 2016.
My suggestion, get as much graphic card power as possible within your budget, then CPU and memory.