@Andreas_Wolf: excellent detective investigation.

According to CPC945 user manual Table 12-5, PLL1 Clock Settings.

Table shows that CPC945 is capable upto CPU 2.664 GHz with ratio 2:1,
Unfortunatelly there is not mentioned nothing about "Speed Classification" that you find.
There is another minor thing, the CPU busses are double data rate. So for one MHz are two data transports. Bus frequency in Mac specs (everymac.com) should be Bit rate frequency.

Anyway, thanks to Andreas we know, that frequency is not big issue as I was afraid. (If in iMac is lower "Speed Classification" than in Powermac, the difference will not be so big, it is the same chip, omly sorted by quality. We are in same situation like with G4 factory tested to 1.5 GHz, overclocked to 1.83 GHz).

Btw, frequency settings of this northbridge is terrible complex, and I don't want to study it complete. It means, there are no direct PLL signals on board (like Pegasos CPU have) and there is probably Apple logic on top of this that controls the frequencies and can be setup by resistors.

And still remains to identify proper resistors. Have the iSight the same resistors numbers like previous models? Maybe if we look on motherboard there can be some small description except the resistor number.

So I suggest to find a volunteer with iMac iSight, willing to burn to dust his iMac, and risking soldering of resistors according to old type motherboard