CPU socket for Mid 2007 MA878LL is Socket P or in Intel specs PPGA478, LF80537, xxxGHz; 4M; 800MHz; SLAF4 or SLAF6Z for the Intel Core 2 Duo Extreme X7800 2.66GHz.
The question is whether the CPU is soldered to the logic board, which in this case it may be possible as the socket description indicates that it is a pin-out type of CPU that is held in place by locking levers. No information has been discovered for a 'guide' to change out this particular iMac 7,1 model CPU. Educated guess is that it is possible, again no info for doing so precisely.
PassMark web site lists CPUs and their ratings. Upgrading from 2.4GHz to 2.60GHz is better by their data than going to the 2.8GHz CPU performance wise. Check it out here:
search all of their pages and click on your model for specs etc. Replacement CPUs on Ebay are expensive for this older performance technology, but this iMac is a very good model. The Mid 2007 accepts Mavericks OS, however the question is whether it can keep up with the newer Software programs due to the older hardware configurations.