Most authorities agree with the 800x600 as a rule of thumb simply because you encompass a broader audience. Since screen sizes vary from one extreme to another, so does screen resolutions. If you own a website the site statistics option offers a good and convenient way of monitoring the screen resolution sizes of the viewers that visit your site. Taking an average over a period of time of these statistics will help you determine what would be the optimum resolution for your particular site. You cant cater to everyones needs but if youre after a larger viewership and not want to limit your site to only a certain sector of the population, then averaging offers a safe point measuring stick.