To paraphrase the famous subtitle of a 1960 Elvis Presley compilation, 50,000,000 Metallica fans can't be wrong. Metallica this week becomes the first hard-rock band-and only the fifth act overall-to top the 50,000,000 mark in album sales since Nielsen/SoundScan began tracking U.S. sales in May 1991.

There were charts before 1991, of course. Billboard first ran an album chart in March 1945. It became a regular weekly feature 11 years later. But precise sales figures weren't authenticated and made public until Nielsen/SoundScan brought the business into the modern bar-code-reading age (for better and for worse).

To mark the occasion, I'm counting down the top 15 acts that have sold the most albums in Nielsen/SoundScan history. Four of the artists on this list are so new that their entire chart careers are contained in the Nielsen/SoundScan era. Eminem debuted in 1999, making him the newest arrival on the list.

At the other extreme, three of the artists in the top 15 released their first

If you had told me at any point after, say, 1992 that New Kids On The Block would one day have another top five album, I would have assumed you were either a die-hard fan, the group's publicist or quite possibly delusional. But that's just what happens this week as The Block opens at #2. It's the group's first top five album since Step By Step topped the chart in June 1990.

Comebacks are always unpredictable, but it's especially hard for one-time teen idols to re-emerge. Their fans have grown up and may be a tad embarrassed by their youthful infatuations. This has hurt a long line of former teen idols, from Bobby Rydell to Bobby Sherman. Among the many others who have found it hard to overcome the stigma of being a former teen star: Fabian, David Cassidy, Shaun Cassidy and, most recently, Rick Springfield.

New Kids' nostalgic, mid-tempo ballad "Summertime" was an ideal lead single from this album. It has solid 505,000 downloads in the past 17 weeks. And three weeks ago, a Greatest Hits

Kid Rock famously elected not to make any of the songs from his smash album Rock N Roll Jesus, notably the multi-format radio smash "All Summer Long," available for download. He has profited handsomely from that decision, as his album has climbed back from a low of #82 in March to #2 this week on The Billboard 200, which is based on sales data collected by Nielsen/SoundScan.

But Kid Rock's decision created an opening for an entity known as Hit Masters to cover "All Summer Long" and release its version as a download. This week, 96,000 fans bought the cover version, nearly as many as the 101,000 fans who bought Kid Rock's album. The Hit Masters version (on the low-overhead Hip Kiddy label) ranks #5 on Hot Digital Songs, up from #32 last week.

"All Summer Long" isn't the only cover version on the current chart. The marvelous "American Boy" by Estelle featuring Kanye West is no longer available for download as a track. If you want it, you have to buy Estelle's album, Shine. As a result,

Jonas Brothers land their first #1 album on The Billboard 200. Their album, A Little Bit Longer, opens with sales of 525,000 copies. That's the highest first-week total by a teen-aged act since Britney Spears' third album, Britney, opened with sales of 746,000 in November 2001. (Spears was 19 at the time.) The brothers' new album sold more copies in its first week than their previous album, Jonas Brothers, sold in its first 16 weeks combined.

Jonas Brothers-Kevin, 20, Joe, 19 and Nick, 15-is the second group to reach #1 that included two teenagers. New Kids On The Block hit #1 with Hangin' Tough in September 1989 when two members-Joey McInytre, 16, and Jordan Knight, 19-were still in their teens. (In a neat coincidence, New Kids' Greatest Hits debuts this week at #22.)

The brothers' sophomore album, Jonas Brothers, jumps from #11 to #10. This is the first time a group has placed two albums in the top 10 simultaneously since another teen sensation, *NSYNC, scored for five weeks in late