Rome wasn't built in a day, but you can work on it all night.

I admit, I hadn't even thought of CAD for urban design. Here's the software in action; CityEngine can be yours for a mere $4950. Of course, you need a pretty mean machine to run it well. Combined with an omnidirectional treadmill, and some groovy goggles, you can take a stroll through Pompeii (as seen on the BBC)