Screen resolution does not equal available pixels for the browser. There are lots of people who don't maximize thier browser to full screen - especially as you get into the larger/wide screens. Some people leave various sidebars and toolbars displayed. Here's a good article on the issue.
http://www.baekdal.com/reports/actual-browser-sizes/
The bottom line is design for 776 usable pixels wide if you want to cover 95% of your visitors. Of course that's based on the data in the article from October of last year from the sites surveyed. It would be really nice to see a current survey of browser viewport resolution for a wider audience. Even better, I'd like to see it tracked in X-Cart instead of screen resolution.