This may help...
I am also using Dell D1728D-LS. I am able to go upto 1280 x 1024.
However the ideal viewing resolution is 1152 x 864.
I am using Screen refresh rate of 60 Hertz
The Driver I am using for XP is Dell VS17X .
This driver is already included in XP.
However XP will automatically select Plug and Play Monitor
so you need to manually change the Monitor Driver.
Good luck..