Singer Lena Horne is considering taking up permanent residence in Washington and has been looking at an 18-room apartment on Connecticut Avenue and at apartments in Georgetown. She has reportedly almost settled on the Connecticut Avenue apartment, in the same building where former senator George McGovern lives . . .

Former Maryland governor Marvin Mandel, who has been doing a radio talk show for about a year now on WNAV in Annapolis, has started doing national radio advertising spots for Cartelco, a car telephone firm. His wife, Jeanne, has just been elected to the board of directors of the Bay National Bank, opening soon in Annapolis, making her what is believed to be the first woman elected to a bank board in the Annapolis area . . .

London doctors have told Princess Margaret, who had part of her lung removed this week, that she should abandon her two- to three-pack-a-day cigarette habit. Chest problems, aggravated by smoking, run in the royal family and had plagued her father, George VI, her grandfather, George V, and her great-grandfather, Edward VII . . . Only in America is there always a way to make a buck. A New York firm, Goldleaf Corp., is buying up all the discarded scrapings and bits of material from the renovation of the Statue of Liberty. That's right, all those rusted rivets and bits of debris will be sold as keepsakes, complete with a certificate of authenticity signed by Chrysler Corp. chairman Lee Iacocca, who heads up Liberty's face lift . . .

Overheard at a dinner party: "The Geneva talks must be important -- we sent Dan Rather, Tom Brokaw and Peter Jennings there. How can the Russians not know how serious we are?" . . .