ATX is a form factor, the size of the motherboard. So, it would depend in your mother boards form factor matches that of the case. You could always just trying fitting in the case and see if the screw holes line up.
From my experience, dell cases are crappy to work with. Many of them are the clamshell design, horrible if you like to work on your computer a lot. Might as well get a new one... if your not picky you can get one from $20-$50, that would look a lot nicer and be much more functional.