There is an earlier article by Dan Knight about CPU upgrades in the 2007 iMac that omits a few possible upgrades. This article is predominantly aimed at helping people get a “Penryn” Core 2 Duo CPU into their Early 2007 iMac, as the chipset does allow several “newer” CPU upgrades.
Apple took a nice step forward when it introduced the first Aluminum iMacs (iMac7,1) in August 1997. The logic board uses the Santa Rosa chipset, and it has an 800 MHz data bus, up from 667 MHz on earlier Intel-based iMacs. The CPU sits in a socket (Socket P), so you can upgrade it!