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).
After quick look to CPC925 user manual is another good news, that CPU bus clock and Memory clock are different clock domains, i.e. changing CPU bus speed don't touch RAM speed.
So, if anybody interested in iMac bus overclocking, open your iMac, check exact northbidge type and revision, find according manual and go boldly forward!
AmigaOS: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep, Sam440ep-flex, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad