The 2007 iMac I'd started on is now in much better shape. As mentioned, I got it free, and it had been sitting in my basement for a while now. The primary thing I knew of was the LCD panel damage, which (far as I know) is what got it retired in the first place. It actually did work, just, it looked like an ant farm or something. Between finding a reasonably priced replacement panel, and realizing that it could run 10.11 (if not beyond!) trying to fix it up got more compelling.
Anyway, I'd posted a pic of the LCD ealier, but I suspect that this was either in part humidity, or
maybe, super-speculation time, trace moisture from the GPU heat pipe causing havoc. Either way, in addition to the LCD damage (which I have yet to see another like), the frame on the front glass had some corrosion, which was on the aluminum as well. Took some scrubbing to get rid of:
It's a little hard to see, but you get the idea.I also got a little overzealous and pulled the mic off of the frame, well, I pulled the wire off and the mic stayed on. So, I had to solder that back together, but hey, it works.
As mentioned earlier, while I was ordering parts, I ended up buying a Core2Duo T9300 on eBay to swap in for the T7700 it came with. To be fair, it was 90% a "for the heck of it" kind of swap, since the CPU was $15 shipped. It's a little performance bump, I believe runs cooler, and includes the next generation's features like SSE4.1 (the CPU requirement for Sierra). However, despite already having had the machine on a fair bit, installing 10.11, and so on...when I tried getting some base numbers to compare to, I saw that the GPU temperature was
waaay too high. I did try turning the machine on for a minute later, and noted that the GPU temperature was high even at boot, and slowly crept up.
So, that was another thing to replace, and I was going to be taking the motherboard out anyway for the CPU. So, I preemptively ordered a GPU heatsink (and, at later points, ordered fresh thermal paste and pads).
Fresh thermal compounds for errrybodyOnce I got (most of) it all back together, the HDD fan - which was previously quiet enough that I didn't even realize it existed - started making a fair bit of noise. After toying with the idea of just unplugging it and maybe swapping in an SSD that wouldn't need it...I relented and took the whole thing back apart. Again. Popped the fan out/apart, examined it, put things halfway back together a couple times for test fittings....
Long story short, one needs to be careful about that fan getting pressure on it. Maybe even just not overtighten screws.
After resetting the SMC (again!) the fans all settled down and it's a nice, quiet machine. With testing, it actually stays quite cool as well. So, it seems like the GPU problem was (probably) tied to the heat pipe, which can apparently leak out its trace amounts of fluid and drastically reduce its heat-transferring capacity. Can't say I've run into it before, but, this is what the internet tells me.
Finally satisfied with how the guts were running, I progressed to putting the LCD back on, then (for the first time in years) the frame and front glass. Did I put the glass on, then notice something that got between it and the LCD, and have to find some suction cups to get the glass back off to clean it? Well, yes, that's basically part of the process in my experience. At least this time, it only happened once!

The other hardware upgrade I ended up doing was trading RAM with my '07 Macbook, to bring the iMac to 4GB, and that down to 2GB. However, since it tops out at 10.7.5...it has been more or less retired. I should probably put 10.5.8 back on it or something.
Then it was time for further testing. I figured, why not a game? Downloaded the Doom3 demo, which is...PowerPC only.
The more appropriate solution came to mind:
For Science.Along with this iMac, you know what else came out in 2007?
The Orange Box! Which, I happened to have gotten for Christmas that year, along with the aforementioned Macbook.
Portal hype was real. My first playthrough of it was on the Intel GMA graphics on that poor little laptop, fans blasting the whole time. I played through it again after I got home, on my gaming rig, which was much better. Haven't really played it since...so I installed Steam, then
Portal, and ran through it again.
While I can't say it ran especially well, relative to even whatever GPU I had back in '07...it was playable, and didn't even ramp up the fans. So, the machine seems to be pretty solid.
The one quirk that I have yet to resolve is basically a given for CPU swaps on Macs - if Apple didn't make a Mac with that CPU, OS X doesn't know what it is, and won't identify it correctly. Supposedly, FakeSMC.kext (Hackintosh stuff) should have fixed that, but, nothing I tried worked. Far as I know, this is basically just a cosmetic issue - specs will claim it's a 400Mhz Core2Duo instead of 2.5Ghz, but
performance is in line with the actual chip:
That's Geekbench on the right, I cropped that information out because I'm smart.It's remotely possible that there are some low power or throttling things that might not work...but it seems to sleep and wake fine. So, good enough really. It's a decent enough machine for basic tasks like web browsing now, and with 10.11 it can still use the current programs. With an SSD it'd be better, but, I was trying not to put much money into reviving it (keeping in mind that these are well under $200 used in the first place). As mentioned earlier in the thread, it
is possible to push these to running 10.12, maybe 10.13 - in this case, the WiFi/Bluetooth would need to be upgraded, I think (or just do without it)...but again, in my experience, 10.12 runs like poop with 4GB of RAM. Sticking with 10.11, the final actually-supported OS revision, is more ideal.