In a sign that the departure of major financial firms from New York City predicted in the wake of September 11 may still become a reality, Morgan Stanley Dean Witter and Goldman Sachs and Co. are planning to shift personnel to new offices outside of the city. Morgan Stanley announced yesterday that it is in the initial stages of purchasing Texaco's old headquarters in Westchester County, and Goldman Sachs plans to have shifted the trading and research sections of its equities division to a new facility in Jersey City by 2004. While the two firms have given assurances that they will maintain a presence in NYC, they both cited a desire to reduce their concentration in lower Manhattan as a motivation in their decisions to add new offices.