I decided to put something of a coda on this thread. Ultimately I decided to leave the 2009 iMac up and running as my work computer, and turn the 2011 iMac into something of a repair project.
I bought 16GB ram, a anti-static wrist strap, PRAM batteries and Arctic MX-4 thermal compound for refurbishing the 2009 iMac. I put in the memory and ran the computer for a few days to make sure it worked fine, then moved onto the refurbishing.
I took apart the 2009 iMac and realized immediately I had the wrong batteries ordered - BR3032 instead of BR2032. That was not a problem though, because I checked the onboard battery and the voltmeter showed it had a healthy charge of 3.3V. I left it in, as it probably is good for another few years at least. The other batteries got returned.
Then I removed the heatsinks from both the CPU and GPU, then cleaned the old thermal paste and applied the new. This was the first time I did this, so was extra careful, and did the thin layer method of applying the paste. Still, I messed up in a couple ways:
First, when removing the CPU heatsink, I turned the the post (with the Philips head screwdriver slot) instead of the Torx screw on the other side of the board. The retainer clip came off the post and went sailing far away into the carpet, never to be found again. Still, I was able to rejoin the CPU and heatsink after re-applying the thermal compound, and I don't ever expect to take them apart again.
Second, on the Graphics card, I found that two types of thermal compound were being used. Apparently I need another, thicker type of thermal paste for the chips surrounding the GPU? Either way I didn't have it, so I used the excess Arctic Silver I had on them so I could reassemble and get it running again at the time.
I would like to know whether using the Arctic Silver on the surrounding chips was a bad idea and I should replace it with the correct thermal compound (and what is recommended), or if it will work acceptably well.
I also noticed something interesting... this was a GPU that had been replaced under the Apple replacement program at the time. I saw that there were several deep scratches on the GPU heatsink pad and traces of even older thermal compound, as it it had been quickly repaired. The old thermal compound was crumbly and cracked in several places -- it was certainly time to replace!
Anyway, I've got the computer back together and running for about 2 weeks now, and everything seems fine. I did have some worrying Bluetooth issues (and oddly higher temps and other weird behavior) at first, but a PRAM reset fixed that -- I remembered I took the PRAM battery out to test it!
Here's what the 2009 iMac temps look like in MacsFanControl while idling with a browser window open:
The CPU temps are pretty decent, and never seem to get over 60°C, even under continuous load, and the CPU heatsink is never less than 7-8°C below the cores. The GPU is a little worrying -- the 2009 iMac apparently can't recognize the newer card's temp sensor, so I can't see the GPU temp; but the heatsink seems to be about the same as before, if maybe a little better.
That's it for the 2009 iMac, unless I need to take it apart again to work on the GPU; I will probably soon turn my attention to the 2011 iMac, which is currently sitting in pieces.
