RSS feed of Post category?

How can I generate an RSS feed of Posts that have a specific ‘category’ or ‘tag’.

This is built in 😀

http://domain.com/category/CATEGORYNAME/feed/
http://domain.com/tag/TAGNAME/feed/

And does it matter whether I apply a tag vs category?

Codewise, no. But read this: http://lorelle.wordpress.com/2005/09/09/categories-versus-tags-whats-the-difference-and-which-one/