Here's how I would start:

1. Create a high-resolution composite image sat map of the area from Google Earth, making sure 1 pixel = 1 meter.
2. Create a heightmap from DEM to match the sat map. Do some erosion and the like.
3. Create basic mask file and ground textures.
4. Import into Visitor and create a .PEW file.
5. Clean up the coastline - always a problem getting that neat.
6. Using the sat map, create a background image for street and building placement reference.
7. Do roads.
8. Do buildings and other objects.
9. Create new sat map for in-game using heavily edited original Google Earth image as a starting point.
10. Refine mask file.

Creating the hi-res sat map from Google Earth is the initial time eater. Use Google Earth Image Downloader which automated the process, few such utilities are available.