Following reports that iTunes has recently become the second-largest music retailer in the United States, researchers are now predicting that Apple's online store is likely to kick Wal-Mart's arse this year. This is mostly thanks to the way Apple provides means of paying for the downloaded content. Wal-Mart currently holds the top spot among its kind.

According to recent data from the NPD Group (a hot-shot global market research company), Apple's iTunes is indeed the second-largest music retailer in the U.S. at the moment, with one to go for pole position - Wal-Mart. The NPD Group's MusicWatch survey talks of last week unit purchases, counting one CD as 12 tracks. Apple has also hit a milestone, as far as its iTunes userbase goes - some 50 million people are current iTunes Store customers.

"We'd like to thank the over 50 million music lovers who have helped the iTunes Store reach this incredible milestone," said Eddy Cue, Apple's vice president of iTunes. "We continue to add great new features like iTunes Movie Rentals to give our customers even more reason to love iTunes."

As for NPD's most recent predictions, Russ Crupnik, president of music for the NPD Group, reckons that iTunes is catching up to Wal-Mart, and fast at that: "Digital sales were up close to 50 percent and CD sales were down 20 percent last year," said Crupnik. "Even at half that growth rate in digital sales, Apple will in all likelihood catch Wal-Mart this year."

"Why?" you're probably asking yourself? Well, because Apple knows how to get the cash from teens, that's why? In providing iTunes Gift Cards, which can be purchased at several retailers, Apple makes it possible for teens without credit cards to pay for their downloaded music via iTunes.

iTunes has sold more than four billion songs up to date. The service offers the world's largest music catalog, which roughly sports six million songs in total.