Cases & motherboards follow "form factors". What model Dimension do you have? If it's 3 years old then you probably have an ATX form factor, which means that, when looking for a motherboard, make sure it carries the ATX moniker. There is a newer form factor called BTX that swapped around the layout of components on the motherboard and has some strict guidelines for fan placement/airflow/etc., but I'm pretty sure it wasn't widespread (or even around) 3 years ago.
EDIT: something else to think about is that you will be swapping all your old equipment to this new motherboard. Some components may not be compatible, depending on what you get- RAM, hard drive interface, and video card specifically. It all depends on your current model. I can try to look up your model in our premier support documentation through Dell and see how bad it will be.:)
EDIT: something else to think about is that you will be swapping all your old equipment to this new motherboard. Some components may not be compatible, depending on what you get- RAM, hard drive interface, and video card specifically. It all depends on your current model. I can try to look up your model in our premier support documentation through Dell and see how bad it will be.:)