Intel iMac CPU Swappable?
However, the author is not sure that it will run completely normally with an upgraded CPU. The author purchased a 2GHz T2500 from a PC shop and installed it in the socket of a 17" iMac and it booted OK. There seemed to be no problems and the About this Mac pane reported a 2GHz duo chip.
The cost of such a minor upgrade ($480 for a 2GHz chip) doesn't make sense at this time but does offer more of a upgrade path than previously available on the iMac.