Custom template for a certain category AND single posts in that category

Hi, this is what I want to happen.
When you go to http://plpnetwork.com you can see what the styles look like. We are adding a new section to the site which will be at http://plpnetwork.com/voices. We’re making a Parent category and we’d like that category plus every single post under that category to show a different design. It will have to pull a new header file as well, we want it to look like this:

http://myaimistrue.com/images/Index.jpg

Anyone know how I can accomplish this? I know I need a custom category template that will call to a different header.php, but how do I make a second single.php template that displays when only Voices category posts are pulled up?

If anyone can help with some markup that would be awesome. Thank you!