I run 1600x1200 on all my comps, but this forum of course is full of people who use more of there computer than the average person. Average web size? I'd go with 1024x768 for the internet browsing category.
I can't imagine running a desktop at only 1024x768, I know that is the most common resolution for gaming, but its just too small for a desktop. You gotta have at least 1280x1024.