most of the users (86%) use a resolution equal or greater then 1024x768: [w3schools.com...]
If you want to restrict to a particular width, I'd base it on web analytics data - to ensure I was not cutting out or putting off visitors that historically make sales.
you're trying to force a page size when the potential user could be viewing the page in a variety of different settings. I'm just pointing out that at 100% page width you cover everyone no matter how they are viewing it...
If it was set at 1000px (for example) the chance that something important might be hanging off the right side of the browser and never be seen exists.
[edited by: Terabytes at 8:33 pm (utc) on Jan. 14, 2009]