From Wikimedia. Also posted to her new Subalbum here.
The dress is showy, but the focus of this ensemble is the elaborate ornament on top of her elaborately curled hair.
Keywords: 1606, Pourbus - Frans the Younger, Margherita Gonzaga, Margherita Gonzaga di Lorena, Marguerite de Gonzague, Marguerite de Mantoue, duchesse de Lorraine, Gonzaga family, Lorraine family, Duchess, high curly coiffure, jeweled feathered ribboned headdress, earrings, tabbed fitted bodice, high enclosing neckline, lace-edged wide deep neck ruff, supportasse, tabbed hanging sleeves, bows, lining, false sleeves, ruffled cuffs, jeweled bodice ornament, basque waistline, farthingale, saya