It's an old story by now: Another album debuts at No. 1. This time it's Garth Brooks' newest, Ropin' the Wind, 2 million copies of which were shipped to record stores even before its release. Such success for a country ''hat act'' shows country music moving strongly into the pop mainstream. Still, No. 1 debuts have become a way of life on today's charts. The No. 2 and No. 3 artists this week, Metallica and Natalie Cole, both debuted at No. 1. LAST POP ALBUMS WEEKS ON WEEK CHART

1 - Garth Brooks 1 Ropin' the Wind, Capitol Nashville 2 1 Metallica 5 Metallica, Elektra 3 2 Natalie Cole 14 Unforgettable, Elektra 4 5 Color Me Badd 8 C.M.B., Giant/Reprise 5 4 Bonnie Raitt 12 Luck of the Draw, Capitol 6 6 Boyz II Men 18 Cooleyhighharmony, Motown 7 10 C+C Music Factory 38 Gonna Make You Sweat, Columbia 8 9 Michael Bolton 21 Time, Love and Tenderness, Columbia 9 11 R.E.M. 27 Out of Time, Warner Bros. 10 3 Rush 2 Roll the Bones, Atlantic

CLASSICAL ALBUMS

1 1 Carreras, Domingo, Pavarotti (Mehta) 53 In Concert, London 2 2 Hadley, Anderson, Green, Ludwig (Bernstein) 5 Bernstein: Candide, DG 3 5 Chicago Symphony (Barenboim) 17 Corigliano, Symphony No. 1, Erato 4 3 Kronos Quartet 33 Piazzolla: Five Tango Sensations, Nonesuch 5 4 Vladimir Horowitz 73 Horowitz: The Last Recording, Sony Classical 6 8 Carreras, Domingo, Pavarotti 9 Favorite Arias by the World's Favorite Tenors, Sony Classical 7 9 Dawn Upshaw 5 The Girl With Orange Lips, Nonesuch 8 6 Dmitri Hvorostovsky 17 Russian Romances, Phillips 9 7 Midori 19 Midori: Live at Carnegie Hall, Sony Classical 10 13 Keith Jarrett, Michala Petri 11 Handel: Sonatas, RCA