Normal maps are expensive. A 2048 x 2048 normal map takes 22 MB of VRAM! So make sure you get every bit of image quality you can out of it. Two things to consider:
- Normal maps are uncompressed (because texture compression really screws up the normal map). So per-pixel detail will be preserved. Use it!
- VRAM is allocated for an alpha channel whether you use one or not. This is because the cards need 32-bit pixels for performance. So include an alpha channel in your normal maps and use it to modulate shininess; this can help create the illusion of different materials.
For scenery objects, do not include an alpha channel if you don’t need it. When textures are compressed, the alpha channel does take more VRAM, and X-Plane will hit a faster rendering path for textures with no alpha channel.