It is easy to fake the CPU frequency in the device tree and make the installer happy. Just copy the following code into a file called fastG4.txt, place it in the root of the primary partition on your master drive (originally Macintosh HD, where the System folder usually resides) and reboot into the Open Firmware monitor (holding down Command-Option-O-F).
\ Claim first G4 runs at 867 MHz, and boot from CD/DVD: dev /cpus/PowerPC,G4@0 d# 867000000 encode-int " clock-frequency" property boot cd:,\\:tbxi
After inserting the installation DVD execute the new file at the command prompt using boot hd:\fastG4.txt.
The use of a file is only intended to ease the command entry, especially when needed more than once, i.e. the code can be input directly at the prompt as well.
This might also be helpful in testing other software excluding slower CPUs.
I don't know if this is sufficient for a dual CPU system. Repeating the sequence of line 2 & 3 with
@1 instead of
@0 will set the property for both CPUs.
Note that the device tree is only a volatile inventory of the hardware found by the Open Firmware during the system start sequence, so these entries will be reset at the next reboot, and (before somebody gets this wrong) we are not talking about overclocking here at all !
PS: A modified file can be used if the Mac OS X Install DVD can't be booted from the built-in optical drive.