You'll have very little chance of replacing it without major surgery to the case and you would also need a new power supply(which would take more surgery).
Depends if Dell used a ATX board or other standard type of board originally in the build. If they did then you can swap it out and shouldn't have to many issues, but most times Dell uses their own custom built boards, so the connectors don't always match up properly.
This kind of workstation motherboard is always customized, also does the computer case.