TANK GIRL: Spring's officially here -- OK, not according to the calendars, but according to all those cotton Marc by Marc Jacobs dresses that have popped up recently (with more to come, they tell us). Generating an almost-bizarre combo of casual and cutesy, this tank-top dress ($198) mixes a ribbed navy top with a fluffy silk skirt (mustard yellow with polka dots, of course). Yeah, the blend of materials is a little wacky, but not so wacky you can't wear it down M Street. Pair it with a long cardigan until the humidity really gets going. GET IT:Urban Chic, 1626 Wisconsin Ave. N.W.; 7126 Bethesda Lane, Bethesda.