If it's the regular ATX form factor you should have enough room to install a card. But you are going to need a power supply that will give your pc enough juice to run all of it's components. If you plan on installing a card, shoot for something 400 watts +.
Antec, Seasonic, Corsair, and XFX make good power supplies. Moreso Antec and Seasonic (Corsair is the same thing, usually, just with their name on it)
Depending on what card you want will help you clarify how many watts you'll need in your power supply