Whats the difference between a 1920x1200 and ...

Just about every decent widescreen LCD monitor allows users to configure video scaling settings.
Basically you should be able to configure your LCD to do the followings:

(I'm using the terminology from my HP L monitor so your monitor may use different names for these settings).

a) 1 to 1:
In this mode video is displayed without any scaling. Any none native resolution will be displayed in the center with the appropriate black horizontal or vertical space surround the image. If you display 800x600 video, the LCD monitor will display exactly 800x600 pixels in the center of the screen.

b) Fill to Screen:
In this mode, none native resolution video will be stretched to fill the entire screen and may stretch the video vertically or horizontally if the source video is not using the same aspect ratio as the monitor.

c) Fill to Aspect Ratio:
In this mode, none native resolution video will be stretched vertically and horizontally to fill the screen while keeping the aspect ratio unchanged.
This is the mode that you want to have if you intend to display non-native resolution video.
Before you purchase any widescreen monitor, make sure it can do video scaling for non-native resoution while keeping the same aspect ratio.

As for x vs x monitors, I personally prefer x for the following reasons:
1) More vertical space when using windows desktop. Yes, there are black bars displayed on the top and bottom when viewing 16:9 video or movies but for everyday computer usage, x is better than x.
2) I can play games in x and x without any video scaling.
That's right. Although x and x are non-native resolution on a x monitor, because they either use the same number of vertical or horizontal pixels as x, you do not need to scale the video (assuming that your monitor does not blindly stretch video to fill the whole screen).
If you need to play some older games in 4:3 aspect ratio, you can choose x without video scaling.
And if in some rare situation the game you are playing only support up to x, you can also play the game without any video scaling.

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