I would imagine most people with a 17" CRT or 15" LCD will be running at 1024 x 768. Probably the most common setting these days.
Those lucky enough to have a larger monitor will be running at a higher resolution. However, for pics posted on the web, we always resize to 640 x 480 for the pic size. This minimises download time without making them too small to see with good resolution on the screen. For those with dial-up connections the download time is most important.
Also pics that size, when sent as email attachments, display nicely within the message so you don't have to scroll to see the whole image.