You arent going to want to use a GX 280, its going to be pretty tight fit and being a dell it likely wont take many standard parts. If its the mini tower the cooling is pretty poor, just spend an additional 50 and buy a good case that you will be able to reuse in the future.
I agree with Hunter.
A case is a excellent investment, and it looks to me that the gx280 is more of a office computer case, and like Hunter said, the cooling would be horrible.
I have a Coolermaster Cm690, its wonderful. Lots of vents, and even has holes in the back for liquid cooling, you could probably find one for around $50