It depends on what you need your statistics for - if you are looking for the basics, such as income per state or county, wage and census data, or other information along those lines, check out the Bureau of Labor and Statistics or American Fact Finder.
If you want specific statistics in a certain subject area (i.e. your topic is something like "does coming from an upper class family help a student get better grades?" or "does belonging to the upper class help keep you healthy?"), you should check out the databases for that subject area. Here is the list of Databases by Subject.
In addition, the database Opposing Viewpoints provides information on the middle class. To get to Opposing Viewpoints, start at the library's homepage, click the down arrow next to Library Resources and click Find Articles and Databases, and click the letter O. Click Opposing Viewpoints and do a search for Middle Class or Upper Class in the box at the top of the page.
