BooksOnBoard.com has a pretty good FAQ/help section that tells you what formats work on which devices.
http://www.booksonboard.com/index.php?F=help
In general, the Adobe ePub format works with all Sony Readers, including your 600. If you buy from them, you need to make sure you are getting the Adobe ePub format. They will also let you test download an actual ebook for free to make sure you are doing it correctly (they have a couple of free tester books).