Will the insides of a dell dimension 2350 fit in a Antec Nine Hundred Black Steel ATX Mid Tower Computer Case?

no reason why not... unless Dell decided to mess with the motherboard... big-picture it should work, but you might have small problems like mobo setup, connecting power button/LEDs/USBs... your back panel might also not fit properly (you know the thing that covers your mouse/keyboard/ethernet/USB sockets in the back) but worst case scenario you dont really NEED it to be there... its nice to have it to stop the dust from getting in but I guess the flip side is that it will improve your cooling ;)