While I can't say with certainty, the likelihood is pretty slim that an ATX board will fit correctly. Dell has been using the BTX form factor for years, so before you commit, if you can't otherwise find an answer, as a last resort I would open the Dell and measure the mounting holes, plus the back plate that covers the IO ports for the back of the motherboard. Also realize, BTX places the CPU in a different location than ATX for both acoustic and cooling purposes, so any cooling shrouds built in by Dell may be in the way of ATX components and need modification or removal.