One thing to note, a Core2Duo is not needed for the iMac5,1 firmware flash and the firmware flash does not need a Core2Duo.
Didn't know that. I thought a C2D was required. That's important given that the extra grunt of the C2D is not as important as the extra 1.3GB of RAM, IMO. Still required if you want 64-bit OSs and apps, of course. Given how old these things are, I would just leave in the CD if that's the case as I would not feel the expense worth it.
Will you check out the two on eBay I linked to above. Most eBay listings don't tell you if it's LGA or BGA.
Notwithstanding the above, they are both look like socket M pin. The second one even advertises at used in an iMac upgrade.