Growing faster than a bill from the ICU, the cast of NBC's ER will admit two new staffers to County General this fall: Maria Bello (Mr. & Mrs. Smith), who appeared in three episodes last season as Dr. Anna Del Amico, the scrappy pediatric intern who clashed with George Clooney's Dr. Ross, and English import Alex Kingston, who recently starred in the racy PBS miniseries Moll Flanders. Kingston a.k.a. the soon-to-be ex-Mrs. Ralph Fiennes will play Dr. Elizabeth Corday, a skilled surgeon who runs into trouble sharing the operating room with Eriq La Salle's dour Dr. Benton. ''Her character will come in as an outsider, both culturally and in terms of medical training,'' says ER coexec producer Carol Flint. ''So it'll give us a way to look freshly at the practices our doctors take for granted.'' Were the producers also looking for a way to ratchet up the babe quotient now that Sherry Stringfield is gone? ''Not necessarily,'' says Flint. ''Our ER has always been a very good-looking group.''