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.
Got an old Color Classic, LC, LC II that’s just too slow to keep using? Using an LC III, LC 520, or LC 550 that just doesn’t pack enough power? If so, Sonnet Technology had a solution in the Presto 040 accelerator, which can push these older Macs to Quadra level performance.