Interesting article -
The reason why 800x600 has been adopted as the standard Web page size is clear: Average screen resolutions of Web surfers can easily be obtained. Here are the latest statistics worldwide:

640x480: 7%
800x600: 53%
1024x768: 31%
1152x864: 2.5%
1280x1024: 2.5%
other: 4%
(Source: statmarket.com February 17, 2001 based on a sample size of 50,465,595 Web sites; numbers rounded to the nearest .5%.)
from - http://webreview.com/2001/03_16/weba.../index01.shtml