The thrust of the research which I have been pursuing is to automatically create these levels of detail. Since you're here, you might be interested in papers that I've written. Face Cluster Radiosity by Andrew Willmott, Paul Heckbert, and Michael Garland. Eurographics Workshop on Rendering, June 1999. Simplifying Surfaces with Color and Texture using Quadric Error Metrics, by Michael Garland and Paul Heckbert, IEEE Visualization 98.
Free software is now available.
Survey of Polygonal Surface Simplification Algorithms, by Paul Heckbert and Michael Garland. The final revision will be published as a CMU-CS Technical Report. A color plate is included separately. The whole paper is also available as a PDF file. For more information, and some free software, try looking here.