hmm it could go either way...
the motherboard MAY be a micro ATX but I couldnt tell without comparing it to a normal mobo.
Looking at the service manual it seems that the front panel connector (lights, buttons and usbs) use a priporitary Dell connection. I think that the back panel isn't replaceable with a non Dell one either.
You could pick up a new case for around 20 quid so it may be best to go down that route.