I know this is an old guide, but just wanted to post that I just upgraded my old 20" iMac EMC 2133 from a 2.0 GHz T7300 to a 2.6 GHz T7800 and it works great. The T7800 was never officially offered in the iMac, but it was offered in the MacBook Pro of the same era, so the OS and firmware seem to know about it. Right now a used T7800 goes for about $21 while the X7900 still sells for $70, so it seemed like a good deal coming from 2.0 GHz. At the same time, I upgraded the original (probably failing) hard disk to a 480 GB SSD. Running 10.10 the machine has gone from unbearable to actually useful for daily work. Pretty awesome for a 9 year old computer and bought me time to save up a little extra to upgrade to a Retina iMac instead of the standard res I would have bought today if the upgrade didn't work. Edit - as requested, here is what my system profiler shows post-upgrade: And here is an image of the iMac all torn apart. This isn't an upgrade for beginners, but it's not really any harder than doing a logic...