"Article in series on executive compensation, Gilded Paychecks, examines two-tier income stratum that has developed in growing number of professions and occupations since 1990's; finds doctors, lawyers and bankers are collecting much larger fees than others in their fields for their work on business deals and cases; finds many moving from academia to Wall Street, and growing number of entrepreneurs seeing windfalls tied largely to expanding financial markets that draw on capital from around world; experts say growing difference in compensation is diverting people from some critical fields; in medicine, where some specialties pay far more than others, nation lacks enough doctors in family practice; doctors have become so interested in business side of medicine that more than 40 medical schools have added optional fifth year of schooling for those who want to earn MBA degree as well as MD; some graduates go directly to Wall Street or into health care management without ever practicing medicine; photos (L)"