There are two kinds of page layouts: floating or fixed-size. This forum uses a floating size which means that no adjustment is needed to view it on the Pandora (except for maybe changing font sizes). Then there's fixed-size pages which typically are 960 pixels wide, e.g. http://pandorawalls.dyndns.org/ (Running on Drupal using the default theme engine with some theme I picked up from somewhere). 960 was chosen because it allows the page to include a small border, and for the browser to include a window border, along with some side toolbars etc. on a 1024x768 pixel screen.
If I were you, I'd either develop a floating size template, or stick to the 960px standard; most modern browsers have a "zoom-out" feature anyways. If you REALLY want to adopt an optimized layout, however, it should be as easy as search-replacing your CSS-file for "960" and replace it with "740". But, as I said, not recommended.
Also, you might consider doing more than just changing the style sheet of your template; the layout is equally important.
Also, it might not be a good idea to start a new site for the sole purpose of getting one optimized for the Pandora's screen size. Get some gimmick or purpose for the site first (maybe you forgot to mention what it was?).
*Also*, NEVER use Joomla! It's horrible! Use Drupal or WordPress instead! I'm serious, Joomla introduces so many headaches because of its weird modularity and is horrible all around in my opinion. It has security issues and very few still use it for new sites. Don't be one of them.
