>> iMac G5 iSight also has U4/CPC945. Besides,
>> U3/U3H/CPC925 is also used in PowerMac G5 AGP ;-)

> This can be carefully checked, if the iSight have full U4 (CP925) and not "light"
> like AGP iMacs. If it is full CPC925 and the same revision used on Quad, it can
> be 1.25 GHz capable (depends on certain version used on certain iMac).

There seems to be some confusion. U4 is CPC945, while CPC925 is U3/U3H.
Changing the ratio on the fastest iMac (2.1 GHz) from ⅓ to ½ would result in 1.05 GHz (as you wrote in comment #19), so testing 1.25 GHz isn't possible this way. What's the maximum frequency a "lite" version of the U4 would be able to run at in your opinion?

> open your iMac, check exact northbidge type and revision

iMac G5 iSight: https://www.flickr.com/photos/htomari/15420568058