AMD GPU
This guide is about setting up NixOS to correctly use your Amd Graphics card if it is relatively new (aka, after the GCN architecture).
Make the kernel use the correct driver early
The kernel can load the correct driver right away (in hardware-configuration.nix):
boot.initrd.kernelModules = [ "amdgpu" ];
XServer
Make sure Xserver uses the `amdgpu` driver in your configuration.nix:
services.xserver.enable = true;
services.xserver.videoDrivers = [ "amdgpu" ];