If it's the Half-Height one like the popular Dell or HP-Compaq SFF, they use a mATX power supply that is smaller.
-It's cheaper to buy a standard mid tower case and transplant all the parts over to it. Then you can use a real gaming power supply and even a full sized gaming card.
There is a Seasonic mATX. There are some cheaper brands too.
-Some Dells and IBM's use a long, skinny version. You will need to figure out which style you have. If it's like a small box shaped one, it's mATX, if its a long skinny box, you will need to find the exact one that says it works with your computer.
-Some, especially ibm sff pcs, use a special board so the cards sit sideways instead of up and down.
If it's that style, you can't transplant them...