The opportunity to live upstairs from famous Harperley Hall resident Madonna is time-limited, since she'll soon be moving across the park to her townhouse. All the more reason to plunk down $15.25 million ASAP for this 11th-floor combo co-op at 41 Central Park West. (A one-unit-smaller version of the combo has been on the market since the spring and is now asking $11.75 million.) The possible combo is a 5BR, 4 or 5BA apartment with 75 feet fronting on Central Park. Fans of Madge, a bit of advice: keep your music collection away from the prying eyes of the co-op board.