Just go to google earth there the whole city is in 3d but we can't use that models. So you pick up a house, bridge etc. and model it. To get the right proportions you can try to get the house in your Sketchup. After you have done a low poly model of your building, you have to do a texture map it should be 256x256 pixel. If you don't know how to do this, I wrote a tutorial how to do it. I think it's one or two pages back.
If you have problems with the texturing, I can do it for you if I have time.
To see how such buildings look like, look at the files topic (http://80.95.161.114/shipsim/forum/index.php/topic,24156.0.html) and download the models from there.
I hope this helps you, if you have any questions, feel free to post them here.
Joey
PS: If you need a proper texture I can post some pictures from my holidays in Venice here.