Look out Wal-Mart, Stevie’s breathing down your neck.

iTunes is now the second-largest overall music retailer — online or offline — in the United States, behind only Wal-Mart, according to research the NPD Group released yesterday.

After Wal-Mart and iTunes, the top music retailers in 2007 were Best Buy, Target and Amazon, in descending order.

As popular as iTunes has become, NPD reported that legal music downloads accounted for just 10 percent of all music acquired in 2007. In addition, 29 million consumers used legal pay-per-download sites last year, up from 24 million the previous year. [Internet.com]