Dyed-in-the-wool Francophiles are singing a loud and enthusiastic "oui" for exclusive Loire Valley tours, being offered June 4- 13 by the French Heritage Society, the American organization devoted to preserving France's cultural riches. Themes range from gardens- secret and royal- to the exquisite niceties of Renaissance court life. Included are luxurious lodgings and rare glimpses of the Loire's most spectacular châteaux. The highlight takes place on June 8: a luncheon at the Château de Villandry, followed by a ball, fireworks, and a dinner prepared by three-star Taillevent chef Michel Del Burgo at the Château de Chenonceau. For information, call 212/759-6846.