The PS3 doesn’t have an ATI gpu. TL;DR: it’s Nvidia.

The PS3 has a weird, one-of-a-kind IBM processor, called Cell. You can think of it kind of as a hybrid design that is both a CPU and a GPU (not like “a chip with both inside” but “a chip that is both”) meant for multimedia and entertainment applications (like a game console). It was so peculiar that developers took a long time before learning how to use it effectively. Microsoft didn’t want to risk it, so they went with a different CPU always from IBM that shared some of the Cell’s design, but without the special GPU-ish parts, and paired it up with an ATI GPU.

Now, Sony wanted to get away with only the Cell, and use it both as CPU and GPU, but various tests showed that despite everything, it wasn’t powerful enough to keep up with the graphics they expected. So they reached out to NVIDIA (not ATI) to make an additional GPU, so they designed a modified version of the 7800 GTX to work together with the Cell. To fully utilise the PS3 graphics hardware, one would have to mainly use the GPU for graphics, and assist it with the special Cell hardware. Which is harder.