Kudos to the modeling details, but man is that excessive for background models. Your computer probably wants to shoot itself. Do yourself a favor and take chunks of background buildings, and turn them into proxy models. If you make 8-10 proxy models of your background, you'll be able to navigate your scene easier, since the models will only load at render time. Otherwise, you'll just bog down your box, and waste time waiting for it to respond when you're trying to work.