Highest OS Version for iMac 5,1?

You should upgrade your ram. Both 10.6 and 10.7 are woefully out of date. The only way I would recommend running either is if your iMac is not capable of running something more up to date. IF you have to run either 10.6 or 10.7 you're going to have to purchase both. Get a 10.6 disc here Mac OS X 10.6 Snow Leopard - Apple update to 10.6.8 here Download Mac OS X 10.6.8 Update v.1.1 and finally purchase an upgrade code to 10.7 here OS X Lion (10.7) - Apple

OR......depending on which iMac you have upgrade your ram from either OWC or Crucial and upgrade to either the free El Capitan 10.11 or Sierra 10.12 if your iMac meets the minimum hardware requirements How to get macOS Sierra - Apple Support

Yes, if you want to erase it, you will need to purchase the retail Snow Leopard disk (unless you still have it), update that to 10.6.8 (free) and either leave it at that or purchase Lion and install that. Unless you are keeping it, I'd leave it at the 10.6.8 rather than paying for a Lion license since that is not transferable, so the buyer of your Mac would need to purchase that himself using his/her own Apple ID.

https://www.apple.com/shop/product/MC573Z/A/mac-os-x-106-snow-leopard

https://www.apple.com/shop/product/D6106Z/A/os-x-lion

So if I only had 2 GB RAM should I upgrade to 7.5 or should I stay with 10.6.8?

AND MOST importantly HOW do I either reinstall the 10.6.8 fresh or Install 7.5, if I have never downloaded the MACOS with my Apple ID and also if I don't have installation disc??

That iMac model can be OS upgraded to OS X 10.7.5 Lion and that is it!

You will need to install 4 GBs of RAM into that iMac.

With that model iMac, only a litle over 3 GBs of RAM will be recognized, but a Mac and OS run better together with two identical sized RAM pairs.

Also, you will need a more recent and secure web browser as these Apple Safari web browser versions are woefully out of date and unsecure.

Download and install.

Firefox 45.9 ESR .dmg install file here.

https://ftp.mozilla.org/pub/firefox/releases/45.9.0esr/mac/en-US/

You may need to update Adobe Flash Player to the last compatible version for Firefox 45.9

You need to use Adobe's Flash Player uninstaller, first, to uninstall the older Adobe Flash Player.

https://helpx.adobe.com/flash-player/kb/uninstall-flash-player-mac-os.html

On the Adobe Flash Player section of the Adobe website, you will need to look for the last Adobe Flash Player version compatible with Mozilla Firefox 45.9.

This is the Adobe Flash Player you are looking for on the Adobe web site.

Adobe Shockwave Flash version 26.0.0.137

Look toward the bottom half of this webpage link.

https://fpdownload.macromedia.com/pub/flashplayer/installers/archive/fp_26.0.0.1 31_archive.zip

Once downloaded on your Mac, dbl-click to open the zIp file

Dbl click to Install the Mac OS version if there is one listed in the archive file (might be a .dmg file).

Good Luck to You!

Yours can only be upgraded to Lion; and, since you only have 1 GB RAM, you need to max that out by going to 3 GB which is all your machine can support. Frankly, I would not be thinking of upgrading an 11 year old machine as any of the hardware can fail at any time - I'd rather save that money towards the purchase of something a bit newer.

http://www.everymac.com/systems/apple/imac/specs/imac-core-2-duo-2.16-20-inch-sp ecs.html

That site is an excellent resource to find the specs for any Mac.

I appreciate your answer, thank you.

But I have the memory, and I just MAINLY wanted to wipe it clean and either reinstall the OS it came with or upgraded to the latest possible OS. It sounds like I can only do that from an actual disc if I were to purchase that disc. Is that accurate ?

Thank you very much for the very quick reply .

How can I tell which is the latest version that my iMac ( or any MAC for that matter) supports?

http://store.apple.com/us/product/D6106Z/A/os-x-lion You can upgrade to 10.7.5 Lion.

this system can run the last version of OS X 10.7 "Lion" if upgraded to at least 2 GB of RAM. Although it has a 64-bit processor, it has a 32-bit EFI and is not capable of booting into 64-bit mode. It does not support "OpenCL" either. Please also note that OS X Lion is not capable of running Mac OS X apps originally written for the PowerPC processor as it does not support the "Rosetta" environment. This model cannot run versions of OS X newer than 10.7.5 "Lion."

Perfect answer, Mike. 😀

Credit belongs to EveryMac.com

What if you hold down the "6" and the "4" keys while booting? Doesn't that cause the machine to boot into 64 bit mode?

Did you miss the part that it is not capable of booting into 64-bit mode?

from here:

http://www.everymac.com/systems/apple/imac/specs/imac-core-2-duo-2.0-17-inch-spe cs.html

Aside from that, 3 GB of RAM (which is all that machine can address even if 4 GB are installed) will not make for a satisfying performance of any OS starting with Lion and newer.