Unfortunately, all we have to work with is Apple's benchmarks of a dual 933MHz G4 with a GeForce4 MX against a dual 800MHz G4 with ATI's Radeon 7500. The Radeon 7500 isn't exactly a next-generation graphics product; there are no programmable pixel or vertex shaders, and only two rendering pipelines. These AnandTech benchmarks demonstrate its Quake III performance to be roughly on par with a GeForce2 Pro.
In Apple's benchmarks, we see the GeForce4 MX beating the Radeon 7500 by only 21 frames per second. Keep in mind that the MX has a 133MHz advantage in processor power, and that the Macs seem to be more CPU-bound in general. This is running on an Apple platform, so there are driver considerations to take into account as well.
So what can Apple's graph tell us about the GeForce4 MX? That it's not a major jump ahead in performance, at least as far as Quake III can illustrate. There will be, of course, more information to digest once NVIDIA makes its formal announcement. Until then, however, these are the only really concrete numbers we have to go on.
Thanks to TR reader Euan Robertson for the tip.