Best way to have full-width no side bars for posts?

Hi Tammi et al.

I'm using the Custom Sidebars plug-in with Simplemarket. I'd like to be able to select when a post (by category, for example) will have a sidebar .. or have no sidebar and be full-width. What's the best way to do this? Do I need to create a custom post type based on single.php? Have you done this before and do you have the code handy?

Cheers,
Mark