Others consider such a small screen area to be impossible to design nicely for, and consequently design for 800x600 - leaving those with 640x480 resolutions to fend for themselves. In my opinion, it's really up to you whether you want to cater to the 640x480 audience, since they appear to be getting smaller with each passing year. If you wish to do so, remember to take into account things like the size of the browser window borders and scroll bars and the like, leaving you a maximum canvas width of slightly less than 600 pixels. What looks to you like beautiful shades of colours may turn out to be ugly combinations on a different system. This is probably because it is the easiest way to keep content which must be on every page in one location.
Many people dislike framed pages.
Furthermore, the vertical scroll bar on the left frame of many sites also usurps valuable screen area. If you are considering the use of frames on your site, see if you can think of alternatives. For example, you might want to use Server Side Includes to include common content like your navigation bar. If you really have to design with frames, make sure you view your site in a resolution like 640x480 to see if people with that screen area can even navigate or view your site. I think you'll probably be unpleasantly surprised.