tiny voices :
A gt720 is not for gaming. It won't really help him. His CPU is going to limit him too much. Even a high end GPU won't help.
Every GPU uses the same motherboard connection.
I'm sorry, but he won't be playing modern games on this PC.
Saying that he won't be playing modern games isn't very helpful!
The problem with the graphics card is that the guy at Micro Center sold me a quiet fan-less one instead of one with a fan, the air vents were too wide to fit in the space provided. I didn't know there was a difference. I got the replacement video card (with a fan) installed and he has noticed a significant improvement in his game play. My nephew is extremely happy with it. It took a few days of learning, but this uncle did good. He's happy, so that is money well spent.
I came here hoping to save time, but if I can figure out this stuff on my own, I guess there's no need to come back here. More optimistic help would have been appreciated.