Quake III Arena: 1024x768
The memory bandwidth of a GeForce2 MX will already be exhausted at 1024x768 and 16 Bits per pixel. You will not be able to get better frame rates when upgrading the processor, but only by getting a faster graphics card. You can also see that the difference between an Athlon 1000 and 1200 is not too important here, as the frame rate is fast enough anyway.
Quake III Arena: 1600x1200
As expected we can see the same kind of limitation with the GeForce2 MX. This time, none of the graphics cards is able to provide enough video memory bandwidth to enable higher frame rates. At 1600x1200 the frame rate depends almost completely on the graphics card. If you take an Athlon 900 you will definitely have a processor that is fast enough for power gaming. A faster processor will not give you any benefits.