Or use Google Sketchup!
There's collections of models you can download to quickly create a city.

Two great city maps done in Sketchup:
http://sketchup.google.com/3dwarehou...8&prevstart=12
http://sketchup.google.com/3dwarehou...1497a98710709d

The last map is kinda cool as the creator has also created a ton of models you can use to populate the city:

http://sketchup.google.com/3dwarehou...6690&scoring=m