You will not be able to use Barnes & Noble or Amazon to purchase books.
Amazon uses a DRM scheme for Mobipocket format books than cannot be read on Sony Readers, unless you wish to remove the DRM then convert the book to a format readable by Sony.
Barnes & Noble will soon be using a epub but their DRM is not readable by Sony Readers.
The Sony will read formats LRF, LRX (sold by Sony), PDF (with and without DRM), Epub (with and without DRM), TXT and RTF. Ebooks for the Sony Reader can be purchased from Sony, Books on Board, Ebooks.com, Powells.com and many others. If you are in the U.S., check to see if your local library supports ebook borrowing.