"I paid $400,000 for that name in 1973, and I'm not giving it up," said Tony Greco, owner of the name "Rive Gauche" and now of the restaurant named Maison Blanche-Rive Gauche, which is often called just Maison Blanche. The confusion has died down a bit now, but began a few months ago when Michel Laudier--chef of Rive Gauche when it was at Wisconsin Avenue and M Street and then operator of Rive Gauche in the Georgetown Inn--wearied of paying Greco $1,000 per month to "rent" the Rive Gauche name and its 25-year-old phone number at the Georgetown Inn address. So he renamed his place Les Ambassadeurs and gave up the telephone number. Some Laudier patrons have told us of the following scenario when they have tried to make reservations at his restaurant. A patron will look in his black book and call the old Rive Gauche number (333-6440). A tape recording tells him that the number has been changed and that the new number is 842-2263. So the diner in search of Laudier's establishment calls that new number, and a member of the Maison Blanche staff answers it with the words "Rive Gauche." When the caller asks where the restaurant is now located, the staff member says 1725 F St., the Maison Blanche address. Some diners in search of Laudier's food have wound up far from Les Ambassadeurs.