Hidden away in the Windows 98 thread (support for the editor in Win98 is discontinued in AGS 3.1.1, by the way folks!) is an interesting bunch of statistics about OSes and screen resolutions that the AGS Editor runs on:
Windows XP: 74%
Windows Vista: 22%
Windows 2000: 2%
Windows Server 2003: 2%
Screen resolutions:
1024x768: 14%
1152x864: 4%
1280x1024: 28%
1280x800/960 (widescreen): 19%
1440x1050: 4%
1440x900 (widescreen): 9%
1600x1200: 4%
1680x1050 (widescreen): 16%
Higher than 1680x1050: 4%
4:3 resolutions: 56%
Widescreen resolutions: 44%