Changing your theme to a variable width theme is simple:
1. Go to your theme subdirectory and open style.css in an editor (notepad, notepad++, etc).
2. Once your .css page is open, scroll down to the section called: #Main { or #MainWidth {
3. in the area: width: 775px; (eg) Change 775px to either 90%, 95% or 98% . And the width will say: width:95%; for example
4. Save the file, clear the site cache and your browser cache and away you go..
The reason why this works quite well is because the templates for visitors, etc, are pretty well created with variable widths and are only being restricted by the #Main { command in the style.css sheet. If not, you may have to do some experimenting with the other widths to get what you want.