You don’t need to go mad, a GTX 960 upwards/newer gets the highest graphics detail in Zwift, and for 4K60 a 1060 upwards is generally fine. As a direct comparison (ignoring the OpenGL performance penalty), a tier of card like the 5700XT is miles higher than required for maxing out Zwift.
The CPU is arguably the more important component, because it determines the frame rate when it’s busy.
