I think that sites should follow a fluid-on-demand layout. By this I mean the layout should be at a fixed width at less than 800px to allow for smaller resolution visitors, but allow the site to be expanded in case something large, like an image, can be posted, and the site doesn't look like crap when something large is posted.

But, again it depends on the site's needs. A forum for graphics and the like should be able to do this because images get large. Just my $0.02.