Wall Street tumbles on GM woes, banks
* Nasdaq composite hits 6-year low
* General Motors falls 15.5 percent to under $2
* The Dow Jones industrial average dropped 281.40 points, or 4.09 percent, to end unofficially at 6,594.44. The Standard & Poor's 500 Index slid 30.32 points, or 4.25 percent, to 682.55. The Nasdaq Composite Index tumbled 54.15 points, or 4.00 percent, to 1,299.59, its lowest close since March 2003.
(Reporting by Rodrigo Campos; Editing by Jan Paschal)