I think this is perhaps a really simple question, and I'm not sure if it's a WP question or an Atahualpa theme question.

Right now, if you click on any category on my page, it takes you to a list of posts with the newest at the top and oldest at the bottom.

I would like it to be the oldest at the top and newest at the bottom.

Is there a simple way to change this?

gen

using wp 3.0.1
using atahualpa 3.4.9

Take a look at this plugin. It may do what you want.