if your computer looks like this
then I believe that is a micro ATX case and the components should fit in any standard case that size. Make sure that the IO shield is removable from the existing case as you'll need to fit it in your replacement case. Some Dell motherboards have screw holes in weird places but usually it's not too difficult to make it work. Full ATX will also work but there will be additional space inside the case.
Post the complete specs for the system and what you intend to do with it and we can probably recommend upgrades more accurately for you. Is it an older computer? If so you have to decide if it's worth putting a lot of money in to.
One good upgrade besides ram is an SSD hard drive, as they offer a great performance boost and you can always take it with you to a new computer if and when you decide to upgrade completely.