Hard to generalize, Oli, but...
The Upper East Side of Manhattan was a borderline slum until the mid-Fifties, when the city tore down the Third Avenue Elevated line, which had blighted the entire area. That stimulated a huge boom in high-end apartment and condo building. Then it became the chi-chi neighborhood of NYC.

If by "lower West Side" you mean, say, the Battery to Greenwich Village: Battery Park City, the major housing development since the Seventies, seems to have recovered from the effects of 9/11. The rest of the lower West Side is pretty stable, except that most of Little Italy has been absorbed into Chinatown. The upper West Side is, as ever, a haven of old apartment buildings and various ethnic intelligencia.

Into the Sixties, Germans were the predominant ethnic group in the Upper East Side (Yorkville), while Central European Jews were their counterpart in the Upper West Side. Greenwich Village's core was predominately Italian. Those are very broad generalizations. The biggest trend throughout NYC is that the real estate boom gentrified just about every former "slum." Every tumbledown six-story building in almost every neighborhood was turned into a condo or co-op, fetching incredible prices. Money broke down all the ethnic distinctions--if you can afford to live in Manhattan, doesn't matter who you are; the only color that matters is green. Despite the burst real estate bubble, it still costs a fortune to live in Manhattan, and plenty of places in Brooklyn that used to be considered seedy. The city-owned housing projects are about the only real estate that haven't been affected.