Or just check teh "Available Templates" option under the Joomla! tab here at rockettheme.com, in the "Features" list if it says "fluid" or "liquid" capable that means full width capable.
My personal preference is for a fixed layout suited to a 1024x768 monitor.
However I have received lots of comments from users about 'wasted space'.
I guess this is since widescreen monitors became popular.
I know it's always up to the designer to decide,
but are there any best practice recommendations for width ?