I would like to point out another way to save memory while making large, detailed worlds. I can't remember the name of the technique, but there is an algorithm that causes very distant objects to be rendered in simple fashion, and gradually become more complex as the player approaches them. The idea is that it is pointless to have these very distant objects finely rendered, since the human eye can't discern the difference in detail at long distance. (few people have better than 20/20 vision) It's a fairly new technique, but it was used pretty well in Halo: Reach to make some impressively large maps. (Forgeworld alone was ginormus for a FPS mutliplayer map, and large even when compared to campagin levels for example)