The London-based Arditti Quartet is one of the most engaging and compelling interpreters of new music, and so is Ursula Oppens, the Harvard-educated pianist whose command of both the traditional and contemporary repertories is simply awesome. What's more, judging by their recent separate appearances here, both Oppens and the Arditti are at the top of their form. Starting this year, they've embarked on a three-year joint venture, sponsored by the New York Composers Forum and Chamber Music Chicago, to introduce six brand-new piano quintets. The first pair is in: One Atmosphere, for Ursula Oppens, by jazz saxophonist Julius Hemphill in his first attempt at "classical" writing, and Last Spring, by the Macao-born Bun Ching Lam, a young woman who's come up with a distinctive, stripped-down, harrowing style. Also on this program, sandwiched between the two quintets, will be a solo cello piece (by west-coast prof Roger Reynolds), a duo for violin and piano (by Elliott Carter), a string trio (a 1988 piece by the talented Russian composer Sofia Gubaidulina), and a string quartet written by John Cage in 1950. Monday. 8 PM, Civic Theatre, 20 N. Wacker; 242-6237 or 663-1628.