Use a different frontend product template for products in a certain category

[Harney]

Displaying a different product template is very easy to do. But you need a "trigger" in order to know when to switch the template. In your case the trigger is the categoryid.

Thankfully the main category id for the current product page is stored in the $product array. So you can switch templates based on some code like this:

{if $product.categoryid eq '243'}
  Do this
{else}
  Do that
{/if}

A brief visual overview of how to determine what variable to use can be seen in this tutorial: http://www.youtube.com/watch?v=lSb0dBP6I98