Phew, let me think, it's been a while since I did any of these. Actually, as I remember it the polygon count wasn't too high on this one. I did make seperate passes for Diffuse, Ambient Occlusion, Key Light and I think a Top Light pass as well, and composited all those in Photoshop, together with a fog pass to give it some depth. The foreground buldings I think were a seperate render. The sky, clouds and smoke and such were painted in PS. There's some great tutorials on the web, and in Ballistic Publishings two Matte Painting books, expensive but well worth the money if your serious.
Ironically I have a much more powerful 64 bit machine now which could handle a lot more than what's in this scene. Maybe I'll get back on the matte painting wagon again and see what I can come up with.