A gasp rose in a Union, S.C., courtroom today when the prosecutor handling the Susan Smith case said he would seek the death penalty. Smith is charged with double homicide in the deaths of her two young sons. The young mother admitted in November that she had pushed her car into a lake with the toddlers strapped inside. In court today, Smith sobbed as the murder charges were read aloud, then returned to the defense table as her lawyer said she was not yet ready to enter a plea. (Judge Costa Pleiconas, who set the trial date for April 24, entered an innocent plea on her behalf.) If Smith is found guilty, prosecutor Tommy Pope said, he'll ask to have her executed in the electric chair.