to clear the texture thing up:

There is absolutely NO memory difference when using external textures. But usually it's better to use external textures, because they are easier to change and you can share the same texture between different models.

A texture that is stored internally requires more memory than the same texture externally when the external texture is compressed (pretty much every format other than .bmp or .dds)
Besides that, textures are never loaded uncompressed. Wether you use a 512² 84kb .png-texture or a 512² 1024kb .bmp-texture - it doesn't make any change in the memory usage because the texture is always converted to an uncompressed format (dds) when loaded by the engine.

The ONLY way to use compressed textures is using the .dds format with texture compression because dds is the same format as the textures that are stored in your vram. However, lossless compression is not possible.