The generally accepted industry standard for GUI/web development is 800x600. It has been at this level for a long time and still is.
All new computers in the last few years usually support at least 1024x768 but it seems until recently, they were shipped from the manufacturer at 800x600. Nowaways, OEMs are now shipping this machines pre-setup with 1024x768 which is much better because it is the first thing I have to change with a new machine.
Give it a few more years and very few will run 800x600. I think too many people do not know about the display settings or there are still too many 15 inch monitors or old machines out there.
For the moment, it seems that we have to stick with 800x600.
How about starting a campaign to get everyone to switch to 1024x768......