Edit that to:

the QS 2002 dual 1 GHz is nearly the fastest stock Mac running 9 natively.

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My bad, or are you saying I can replace my Dual 1 GHz QS 2002's processor with the Dual 1.42 GHz processor?

Is this only possible for the MDD's Dual 1.25 GHz processor?

The Apple Power Macintosh G4 1.42 DP (FW 800) is a Classic Mode Only* box.

The Power Macintosh G4 (Firewire 800) series is similar to the "Mirrored Drive Doors" series that it replaced -- both share an identical case design complete with "mirrored drive doors" and the same bus speeds (133 MHz or 167 MHz) and RAM (PC2100 and PC2700), for example -- but the "Firewire 800" models have faster optical drives and add a single Firewire 800 port along with support for onboard AirPort Extreme (802.11g) and Bluetooth 1.1.

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It looks like MDD switched to the Classic Mode Only* Xserve architecture at the Firwewire 400/800 cusp.

My guess is that the Procs are not compatible across the architecture void . . .

. . . but I'm sometimes wrong about my ASSumptIons! :-/

I did my "fastest Native 9 Box" research a few years back, but ISTR that a Dual 1.25 GHz Box was the limit.