There is a very quick and easy way to generate any number of interesting cityscapes using any photo editing or graphics program on any operating system, which is not processor hungry. Here is how:

We are going to make a bump map to use in Bryce.
First, create a square canvas, say 800x800 pixels, and color it black.

Second, create a whole lot of squares, rectangles, ovals, circles etc using DIFFERING shades of grey of differing sizes. Imagine you are looking straight down on the city grid from the air. See the attached example. If you seek street grids, make sure you leave black spaces for these in your layout. Save the image as a TIFF or good quality JPG.

Third, open Bryce, create a terrain, click E to edit the terrain. Then click the Pictures tab in the Editing Tools window. Load the image you just made and click Apply.

You now have a terrain which consists of a whole cityscape full of building like structures of varying sizes and heights. You can play with some of the Elevation settings to vary the height and appearance, some smoothing is recommended to create more rounded edges etc.

Click the tick to save the terrain back into the main Bryce window. Position into your scene, apply some kind of convincing city texture to the terrain, add atmosphere, sky, and even some ground lighting, and there you have it.

Adjusting the height of the terrain in edit mode changes from Manhattan style to country town.

Obviously not a replacement for more sophisticated modeling and texturing of individual buildings, but it can look surprisingly good as a distant city, or a city from the air viewed at an oblique angle.

Best of all, the terrain is reasonable fast to render, although applying a complex texture to the terrain with reflectivity and or metallic properties will substantially increase the render time. I will post an example city scape created in this way onto my gallery page here for you to see.

I hope this helps out newbies, or even experienced Bryce users who have never considered this instant Metropolis method before. Please feel free to save the enclosed city bump map to get you going if you would like to.

See example images here:
http://www.renderosity.com/mod/gallery/index.php?image_id=1280792