Surely your site should resize to the screen? Who wants to have a screen where either the content doesn't fit, or you have two great big gaps on either side? Either way the designer is not catering for the viewer and in my opionion isn't doing their job properly. Further to this consideration, if the site is designed properly and uses CSS, the viewer can choose to change the font size for example without messing up the format of the page. If you look at the WAI (Web Accessibility Initiative (WAI) - home page) they are very clear "Use CSS for layout and style where possible."; in doing so, the user will have some control over how they view your site, otherwise it could be unreadable. Just visit a few of your favourite sites using Firefox, press ctrl + a few times and see what happens, probably an undreadable mess. The only exception I would concur with would be that 640x480 is probably not worth bothering with, but then again, if its designed right then your site will fit anyway.