The US financial system could be in for its biggest shake-up since the Great Depression, after comments made by two of the most powerful figures in the United States economy.   Federal Reserve chairman Ben Bernanke and US Treasury Secretary Henry Paulson addressed a Congressional panel on "Systematic Risk and Financial Markets" Thursday to argue a case for a new system of regulation.   The idea that markets are best left alone has been the financial creed behind the US economy for decades, but the near collapse of investment bank Bear Stearns and the continuing credi