Did you install a driver for your new monitor as well? If windows sees it as the default Plug and Play monitor, it won't report the supported resolutions to the game properly.
I just installed the drivers for the monitor and it is now not listed as Plug n Play. However, Sins still doesn't show the correct resolution in the list.
BTW - It's a Samsung T240HD LCD widescreen.
I just checked and it is running at the correct resolution so I guess it works I just was curious as to why it's not showing up as an option in the menu.