Well, Sirius Satellite put together a great little site that allows you to enter your Zip code and see which stations should render the best reception in your area.
Keep in mind that this could be particularly useful information when traveling. If you’re going to take your iPod and transmitter on a trip why not look up the zip codes you’re going to be traveling to or through and print out the recommended frequencies for those areas. This will prevent you from having to search and search every time you hit a different city.