When I was building my first websites, way back in the last millennium, the most common screen resolution was 640 by 480 pixels. Most websites at that time were being designed for 800 x 600 screens, which was rapidly becoming the standard.
Desktop, and laptop, resolutions have been climbing ever since. If netbooks hadn’t become popular a couple of years ago, it is likely websites would be designed for 1200 to 1400 pixel wide screens. Instead the common design standard for current websites is based on a 960 pixel wide grid, centered on the user’s screen.