Many people lived in Prague before they decided to leave Bohemia - maybe your ancestors were one of them. Of course Prague today is a bit different from the Prague before WWII and it's useful to know that not all streets have the same names and so on.
There is wonderful source of such information as streets and house numbers - it's map of Prague from 1938. It was published by the Municipal Library of Prague on its website: http://web2.mlp.cz/mapa1938/klad.php
When you click on the map in the area which is interesting for you, you will get large map of that area. It shows street names as well as house numbers and information about important places (such as churches, schools, cemeteries, playgrounds etc.).
If you know the street name but you don't know where in Prague it was located, you can use index (Rejstřík) which is available. You just click on word Rejstříky in top right corner and you get a table with street names, both old and current ones, together with links to proper map parts (number in "mapa" column).
I use these maps quite regularly to find out where people were living in the first half of 20th century. And I highly recommend these maps to anyone whose ancestors lived in Prague.