2 NeoStrider, I have a bit difrent workflow.
I create the entire area base surfaces,
exporting parts of it and using those parts as
reference for the details, after that I just import
the detailed parts to the main scene.
So I don't have perfomance troubles while modeling
I would have problem when I have the entire scene
assembled. For now the scene is not as heavy as it looks.
For exemple my poly count (all the detail meshes) is
about 300,000 only, and as far as I know my pc will
give a good performance up until i have about 1-2
milions. The only thing that may affect perfomance
in a very bad way is heavy NURBS triming i have on some
ornaments.