Adding “sort by” options

Hi, uh, me again.
I was wondering if it’s possible to add custom “sort by” options to the listings overview page.
Currently, you can sort by price (asc/desc) and date (asc/desc).
My client wants to be able to sort by make (alphabetically) and category (also alphabetically).
I’m staring at the two files orderby.php and Query.php which look like the most promising candidates for adding this functionality, but I have no idea what I should be putting in there in order to get it to show up and to actually work…

The page I need help with: [log in to see the link]