I did a little research into this when I bought my 9600 pro 64Mb (G4 modified) from OWC.
I have seen direct comparisons between the GeForce 4ti and GeForce 5200. At lower resolutions the 4ti is significantly faster, but as resolutions increase the 4ti framerate drops significantly faster than the 5200 Ultra's, so that they are roughly equal at resolutions at or above 1280x1024.
The Radeon 9600 Pro GPU us roughly equal to that of the 5200 (the PC benchmarks I've seen indicate it is slightly better actually), so I think we can use the 5200/4ti comparison to extrapolate the same conclusion for the Radeon 9600 Pro. With one caveat. The 9600 Pro has 1/2 the VRAM, which may hurt it in some areas. But I think the 4ti's VRAM is slower, which may even things out a bit. If you check the Doom 3 benchmarks I've linked below, you'll see that a 5200 with 64MB beat a 128MB 4ti, so it seems like the extra VRAM doesn't make a big difference.
One BIG factor I've not mentioned yet is that the Radeon 9600 Pro is officially 100% Core Image comaptible, whereas the GeForce 4ti is not. I'm not certain as to EXACTLY what the 4ti can and can't do, but the 9600 is a newer GPU design and can do some things the 4ti can't.
I dug up a 5200/4ti comparison link
here.
Irritatingly, I was neve able to find a single test where Barefeats included both the 9600 64Mb and 5200 64MB. However, Barefeats did do a
Doom 3 benchmark with both the 5200 and 4ti. The 5200 was 13fps faster than the 4ti, so it seems that the newer games choke the 4ti earlier than the 5200. Expect the 9600 Pro's numbers to be pretty similar to the 5200, maybe a tiny bit better if you tweak the overrides with the ATI Displays control panel.