At work I run 1280x1024 so I can see most of mt Excel spreadsheet. At home I run 1280x1024 because I can
and so that the display is the same as at work. When I'm designing web site I usually design them for 800x600 minimum. That way most of the viewers get the look I set out for. I test all my work by changing my resolution. I also check my work with a 15" monitor I have at home.
About .com... A commerce site must be made for 800x600 at least. In my opinion.
My personal site is designed for 1024x768 but it does ok at 800x600.