Fitting A Micro Itx Motherboard In An Atx Case

I was given an case for an HP computer a while back. It seems only one or two years old. It has most of the original case parts within it, along with some wires. Now, a friend of mine has an smaller computer in a Mini-ITX case, and has been looking to upgrade both his power supply and his graphics card. However, most graphics cards don't seem small enough to fit within his case (height wise, at least. He has enough room in both length and width), and there's only a select number of Mini-ITX power supplies on Newegg (and the highest wattage is 400w). Logically, I figured he'd be able to take my extra case and fit everything else in there, but I was unsure if it would fit properly.