I have to agree with the other reviewers. I have a G4/dual 450MHz/512MB. After a recent clean reinstall of OS 9.1 and 9.2.1 upgrade, my 256MB DIMM 'disappeared' and System Profiler told me it was "not compatible," even though it's a PC100-222 module like the 126MB DIMM in slot 1, and even though I was previously running 9.2.1 and the firmware upgrade with no problems. This little utility solved the problem.