The new Old Navy, recently opened at 503511 Broadway, has been greeted with the slavish adulation that attended its arrival on Sixth Avenue (in the heart of the former Ladies' Shopping Mile, another resuscitated neighborhood). A plain cardigan, if not merino then surely its equal, 100 percent lambswool, in a pleasant if predictable baby blue, is $28, and they even throw in an extra button. Unfortunately, once you move beyond the sweater table, the fashion quotient descends precipitously. Still, the mood is very Felicity, if that's your sartorial cup of Sleepy Time tea, and there's nary a pleated skirt in the house, which is either criminal or refreshing, depending on how much shopping you've done this year. If the stock in no way lives up to Carrie Donovan's weekly gushfest in the Times,at least the prices are above reproach.