Dell Xps Studio 435mt, ATX or mATX ?

It is ATX. The upgrade is the best one you can do. The cooling in the "White Elephant" is terrible. I dropped 20C by changing the case. A mid tower ATX case will do the job well. My 9000 resides in a Antec 300 case. Front and back are 120mm fans, top a 140mm. Included with the case. The 92mm rear fan does not do the job in the stock case. Save the front 120mm located in front of the hard drive as a spare. You will need the hardware included (screws & stands) with a new case, Dells stuff is not transferable. The power source is not going to fit and is not standard ATX. It is a fire hazard also. Try to find and purchase a bottom mount PSU case as it will run cooler also. Antec, Coolermaster, Rosewill,and Compucase all offer very nice cases in the 50 dollar range, or less on sale. Link below to shop. Hope it helps.