Gatin the player will be tough, especially since they have to be able to see that there's a city 'out there' and not just this one block.
I would also suggest you base your city on a real one. Most people here do American and British style cities, with lots of brick. However, if you're up for creating some custom media, you could go for something more original, like Eastern European (a la Half Life 2). It also depends on your story; if it's set in America, it's unlikely to have European-style cities.
Once you've decided on your style, make some buildings as static entities, this will save your poly-count a bit. Basically, decide which buildings the player cannot go in to and make an entity for it. A textured cube would do, especially for buildings that the player sees at a distance.
Assuming your level is set in America (and I have never been), I don't know what sort of landmarks an American city has. Generally though, the older buildings are in the middle of the city, there's usually a financial distinct with some skyscrapers and there's more and more green space, the further out you get.
Yes, it is difficult to do what you want to do!
The hardest thing is going to by making sure the framerate doesn't drop too much.
Hope that helped,
AE
