IS there no end to the talents of David Tang? Entrepreneur, auctioneer, pianist, English teacher, commercial model, club owner, cigar salesman, charity worker are just some of the many facets to David that we know about. But now add croupier and entertainer to the lengthening list. For a letter from the American Women's Association informs us that at its Golden Nugget Saloon evening on May 1 at the American Club, David ''will be on hand to perform card tricks and roll the roulette wheel''. Knowing the sheer versatility of the man, probably carrying out the tasks while smoking a cigar.