You can't just go to a bookstore on the Web anymore. Both Barnes & Noble (www.bn.com) and Borders (www.borders.com) now sell music and movies as well as books at their sites, and market leader Amazon.com (www.amazon. com) has turned itself into a virtual Wal-Mart, even selling electronics and gardening tools. A look at the three sites turned up a lot in common--but also unexpected differences.

Amazon.com is still ahead of its competition. To start our comparison shopping, we looked for a popular sci-fi title, Neal Stephenson's "Cryptonomicon." At Amazon, you can buy a hardcover edition of the book for $19.25--but that's just the start. It's just as fun to poke around Amazon as it is to buy anything. If you're a Stephenson fan, you can also get the book on tape, purchase a signed first edition for $75 or enter an auction for a signed first edition, with bids starting at $49. Oh, and thanks to Amazon's Purchasing Circle feature, you can also find out the useless but interesting piece of information that the science fiction book is currently the most popular title among customers shopping from the California-based video game developer Electronic Arts.

Meanwhile, Barnes & Noble matches Amazon's price for "Cryptonomicon," but Borders sells it for slightly more, at $22.

To try some music shopping on the Web, we ran a search for the chanteuse Marlene Dietrich, whose back catalogue is scattered across various labels. Borders turned up 13 Dietrich-related titles ready to ship right away, while Amazon had a total of 19 discs that it could ship. (Both Borders and Amazon also listed several titles available via a special order.) But the online music store CDNow Inc. (www.cdnow.com), pulled ahead of even Amazon, with the best selection of all, at 24 titles. For its part, Barnes & Noble turned up only one title--not a particularly relevant one at that.

Last, we did a little DVD shopping, starting with "The Matrix," one of the top selling digital video discs to date. Amazon has it for $17.99, DVD Express (www.dvdexpress.com) sells the title for $17.49 and both Barnes & Noble and Borders sell it for a penny or two under $15. But when we went trolling for obscure titles ("Disco Godfather," anyone?), we found that Amazon's prices generally beat the other three sites, and that both Amazon and DVD Express carried many obscure flicks unavailable at Barnes & Noble and Borders. Big surprise here: Amazon doesn't make more use of the Internet Movie Database (www.imdb.com), a site owned by the company.

The lesson from our comparison shopping here seems to be that if you're looking for an obscure title, Amazon will have it at what looks like a reasonable price. If, on the other hand, you're looking for a name-brand title, you might save a buck or two (but not much more) by trying the competition.