Would ASUS Sabertooth X58 LGA 1366 Intel X58 SATA 6Gb/s USB 3.0 ATX Intel Motherboard fit in a dell dimension?

Believe your Dell case is for micro ATX. The Sabertooth is ATX,( 3" longer). As far as cases a ATX mid tower is what you need., My Sabertooth resides in a Rosewill Smart case. A Antec 300 sits next to it with another Asus system. The next system is in a Rosewill Wind Knight. A new system going together in a Compucase Blitz. All these will handle your Sabertooth. All cool well and have generous options. There are many cases, these are just a sample. Suggestion: choose a case that has a bottom mount PSU. All above do. Here's a link to shop. You will spend time here, there are 100s of cases. Hope it helps