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Officially the iMac 6,1 can run up to Mac OS X 10.7.5, but with MLPostFactor it will run 10.8.5 just fine. I recommend upgrading the RAM to 4 GB first (3 GB is the usable limit). Anything past 10.8.5 will sadly be problematic, as it isn't possible to get graphics acceleration on Mavericks - El Capitan. Installing a newer Linux OS is possible however.

[Guide] Success! Install 10.8 on old unsupported Mac
 
For mine I would stay with Snow Leopard was Lion was a terrible operating system.

Being what Apple calls an 'obsolete' machine, regardless what operating system you could install, it will no longer be supported by Apple with Security and other updates and I do not like doing hacks just to make things work on what they were never designed for.
 
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For mine I would stay with Snow Leopard was Lion was a terrible operating system.

Being what Apple calls an 'obsolete' machine, regardless what operating system you could install, it will no longer be supported by Apple with Security and other updates and I do not like doing hacks just to make things work on what they were never designed for.
Mountain Lion is actually a decent improvement over Lion despite not being officially supported, and since the OP is asking what the highest OS is that these will run, I think my recommendation is sound. However, if someone asks about a 2007 MacBook and Intel GMA graphics with the original HDD, then I'd probably recommend staying put.

I speak from personal experience also; because I own an iMac 6,1 running 10.8.5, and I use it almost every day. With an SSD it boots up and runs plenty fast; even before with the stock drive it was a little bit slower than 10.6.8, but still usable. The latest versions of Firefox ESR 52 (fully up-to-date and patched), Kodi won't run on 10.6.8. That would limit this older Mac even further for me.

But Apple (and subsequently many third-party developers) have abandoned even 10.8.5, as you said, and forced me to at least try Linux on it. It was never designed to run Linux, and it's not 100% perfect. But I find it to be more efficient than OS X for tasks like modern web browsing and watching YouTube videos in-browser.
 
Thanks for the information, guys. As I only do photos and email, a little web poking about, it seems best to stay as is. I get a lot of "your browser isn't supported" messages, so the plan now is to to buy a new desktop.
 
OP wrote:
"I get a lot of "your browser isn't supported" messages, so the plan now is to to buy a new desktop."

If you're using Safari, this is to be expected.

Have you tried some other browsers?
I'd suggest download and trying these:
- iCab
- Firefox
- Waterfox
- Epic privacy browser
 
Thanks. I know Firefox has fans, works well, yet this browser non-support hasn't really been an issue enough to be bothersome. I'm thinking Mac going over these years from 10.6 to 10.13 now has enough enhancements to Safari, Photos, and Mail, even Siri now, to warrant a new unit, bigger screen.
 
Thanks. I know Firefox has fans, works well, yet this browser non-support hasn't really been an issue enough to be bothersome. I'm thinking Mac going over these years from 10.6 to 10.13 now has enough enhancements to Safari, Photos, and Mail, even Siri now, to warrant a new unit, bigger screen.
Get crazy with a 2017 27" iMac :cool: It is so sweet.
 
OP wrote:
"I get a lot of "your browser isn't supported" messages, so the plan now is to to buy a new desktop."

If you're using Safari, this is to be expected.

Have you tried some other browsers?
I'd suggest download and trying these:
- iCab
- Firefox
- Waterfox
- Epic privacy browser

Don't forget to try Camino and TenFourFox
 
Don't forget to try Camino and TenFourFox
iMac 6,1 is based on an Intel Core 2 Duo, not a PowerPC processor. Regarding TenFourFox on Intel, here's something I posted a little while ago:
TenFourFox is written for the PowerPC architecture, meaning it can only run in slow Rosetta emulation. Sadly there is no up-to-date Intel version.

Since it's based on Firefox 45 anyway, you're better off just going with that. No longer supported as of April this year.
 
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