Marc Jacobs has long been a fashion-insider favorite, and with his collection, shown late Monday night at the New York Armory, he once again proved why. His Spring 2000 line is sure to become one of the most important collections of the season. The mood of the clothes was evocative of a California surfer girl on holiday in a Parisian discotheque circa 1979: canvas trousers with green and red piping were paired with poor boy cashmere T-shirts; Nimes denim pants, sundresses, and skirtsuits looked crisp and fresh with striped gauze tops. The clean and simple lines were thrown occasional accentslike paillette trimming on green cotton satin pea jackets, spotted dresses, and striped trousers. The overall feeling: effortless, understated chic.