The U.S. Census Bureau has a map and a lot of text information that may interest many people who are beginning to research family history. It shows the top reported ancestries in the United States, as provided by the 2000 census. Areas with the largest "American" ancestry populations were mostly settled by some English, French, Welsh, Scottish and Irish; but by an overwhelming majority of Scots-Irish.
Obviously, this doesn't prove that your ancestors settled in these areas after arriving in the U.S. as there were many exceptions. However, it does show many of the more likely locations where you might start your search for records. I also find it to simply be very interesting for most all Americans to show "where we came from."
A small image of the map is shown above. You can click on it to see a larger version. You can also see the same map and a lot of detailed text information about immigration on the Census Bureau's web site at http://www.census.gov/prod/2004pubs/c2kbr-35.pdf.