Eminem [ tickets ]’s “The Marshall Mathers LP,” which for the past eight weeks occupied the No. 1 spot on the Billboard 200 album chart, has been dethroned by a compilation of previously released hits from various artists. The “NOW That’s What I Call Music Vol. 4” compilation, released last Tuesday (7/18), sold over 300,000 copies in its first week, making it this week’s No. 1 album.

“NOW Vol. 4” features 18 previously released tracks by some of pop and rock radio’s most popular hit-makers, including Britney Spears [ tickets ], Backstreet Boys [ tickets ], Blink-182, Hanson and Lenny Kravitz [ tickets ]. It is the first time a compilation of previously released music has ever debuted in the No. 1 spot, according to Billboard.

In a user review about the “NOW Vol. 4” compilation posted on Amazon.com, the comments of one customer who bought the disc last week might explain the reason this latest installment--and previous editions in the “NOW” series--has been so successful.

“The reason that I buy the 'NOW' CDs is because you shouldn't have to change CDs or skip around to ‘the good song,’” writes the consumer. “I know that is why a lot of you buy these kind of CDs, too, and if you are this type of person, ‘NOW 4’ is the CD for you.”

The three previous installments in the “NOW” series peaked at the No. 10, No. 3 and No. 4 positions respectively during their runs on the charts.

Britney Spears, meanwhile--whose song “(You Drive Me) Crazy” from her 1999 debut album is featured on “NOW Vol. 4” album--hung on to her No. 2 spot this week with her latest disc, “Oops! … I Did It Again!”

“The Marshall Mathers LP”--with sales of only about 8,000 copies less than Spears’ “Oops!”--ended its 8-week hot streak by slipping to No. 3. The album still managed to sell another 233,312 copies, however, which brings its overall sales total to roughly 5.2 million, according to a spokesperson for Interscope Records.

Rap-newcomer Nelly’s “Country Grammar” fell from No. 3 to No. 4 this week, while “Infest,” the debut album from rock group Papa Roach, climbed from No. 8 into the No. 5 spot.

Rounding out this week’s Top 10 are Creed’s “Human Clay” at No. 6, ‘N Sync’s “No Strings Attached” at No. 7, “The Nutty Professor II” soundtrack at No. 8, 3 Doors Down’s “Better Life” at No. 9 and the Ruff Ryders compilation album "Ryde or Die Vol. II" at No. 10.