Upgrading logic board to a faster CPU

How about going with this: iMac 24" 2.8 GHz Core 2 Extreme The chip alone is all that is needed here as these are the exact same logic boards.

Review the Intel ARK comparing the specs of the chips between yours and the other processor that fits:

• 2.4 GHz Intel "Core 2 Duo" processor (T7700)
• 2.8 GHz Intel "Core 2 Extreme" processor (X7900)

If your system is working why don't you try boosting the system memory up to the max and switch out your HD for a faster drive like a Seagate SSHD hybrid. Or if you don't need the storage go with a SSD. But you will need to be careful here at this system is only a SATA III (3.0 Gb/s). So the drive you put in needs to run at this speed (review the drives spec sheet before buying!

In any case this would get you more bang for the buck and less risky than swapping out the logic board.