If you're doing top-down buildings then all you need are some building shapes and some roofy patterns. Merge the patterns to the shapes and then you can start rotating the shapes to fit the streets. To get the angle of the roof you just use a bevel/emboss or you could hand-paint the slope with a muted black. As for creating the house shapes, well only your imagination is the limit there. You can check out my house brushes to get some ideas, though...the link is in my signature below.