My dad needs a new video card, but he only wants ATi. His current one is an ATi Radeon 9600. Is an X1600 PCX a good Vid Card Upgrade?

No Not AGP, PCI-X.
What kind of G5 PowerMac? The AGP/PCI-X that came with 9600 with only go up to a 9800 if all he wants is ATI. The PCI Express model never came with a 9600, but those can go up to an X1900. There are flashed PC models, but I don't know what would work.
I have some cards still available if you want one cheap.
PCI-X is just an overclocked PCI card, and those machines still had AGP video card slots.
If it is a PCI-Express machine, I thought those video cards were a little newer and were the Nvidia 6600, 7800, & Quadro cards ... with the ATI 9600/9650/9800/etc. the last of the AGP cards from Apple.
http://themacelite.wikidot.com/compatibility <- AGP OEM/Flash etc. card matrix
Yeah, PCI-X was supposed to be the next version of a PCI slot, while PCIe is the next version of AGP.PCI-X and PCI Express are two different things.