That's what I thought it was supposed to be, but DuckHunt keep coming up at a much smaller resolution.
Do you have 'switchres 1' set in mame.ini?
I'll have to check when I get home tonight.  Which way should it be set?
[no]switchres
        Enables resolution switching. This option is required for the
        -resolution option to switch resolutions in full screen mode. On many
        modern video cards with hardware stretching support, there is little
        performance penalty at higher resolutions, so it is nice to be able to
        get rid of the monitor resync time when you run in full screen mode.
        This is also useful on LCD displays with a fixed resolution. The        
default is ON (-switchres). This option requires -ddraw.