it's not often enough you can take an OEM mobo and put it in a ATX case.
My best suggestion is to look really close at the screw holes of the case and the mobo in the Dell and see if they match up.
If you can't tell by eye, tape some 8x11 paper together to span the screw holes of the new case (trim as needed).
Then take a pen or alike and poke holes in the paper for all the screw holes you see.
Take that paper and overlay it over your Dell as best you can and see if they line up. You may have more screw holes in the paper than the Dell, but as long as you have 6 or so that match, you're good to go.