I know this is an old post, don't know if you've had any success; with the Dell SFF I would check what power connectors your PSU has. I've got a Dell SFF (can't remember the number) and it has non-standard power connectors which prevent a standard mobo being used. And, of course, they are non-standard power supplies so you can't really upgrade them.
I ended up getting a new PSU, mobo and case and then transferred the rest of the internals.