Roughly: 1) 40MB of textures 2) split into mipmaps 3) split into ~32x~32 tiles. 4) streamed in according to camera 5) with the lowest resolution layers always

Roughly: 1) 40MB of textures 2) split into mipmaps 3) split into ~32x~32 tiles. 4) streamed in according to camera 5) with the lowest resolution layers always loaded to handle whipping the camera around quickly. 6) and the Z-buffer off to save memory bandwidth. The video is worth watching too.