I have the previous GDM-W900 hooked up to a Beige G3 with an iXMicro video card. The key to taking advantage of all that screen real estate is to use a widescreen resolution mode.
With the iXMicro card, the following resolutions provide a desktop that takes up the full width of the screen:
1376x768 (1.79 aspect ratio) (i.e 1376 divided by 768)
1600x1024 (1.56)
1920x1080(1.77)
1920x1200 (1.6)
If your video card (Radeon or GeForce) provides any of these resolutions or any others with an aspect ratio of 1.6 or higher, you should be able to display a widescreen desktop on the Sony.
ktpub - if you actually have 1900x1200 available, this is very close to a 1.6 aspect ratio. It's possible that if you have lots of black space on the sides, the desktop is showing as slightly squeezed. You may need to use the monitor controls to horizontally expand the screen (left/right). This resolution may not be one of the factory settings and once you set the width, the monitor should remember it next time.
Once you use one of these widescreen Sony monitors, it's very difficult going back to a regular 4:3 monitor. Also, another benefit is that these Sonys serve as wonderful HDTV displays (two RGB inputs).
Luis