I think 10.7 (Lion) was the first to need 2GB of RAM.
I've got a DMG image of Mac OS 10.6.3 and a couple of DMG files that should step it up to 10.6.8. It's been years since I've played with old iMacs though and I'm not certain that I have all the updates. The DMG is about 6GB and the updates another 2GB on top of that. Not sure if you're interested in downloading all that - and then finding a dual layer DVD burner + suitable blank media. You can make a bootable USB from a DMG file, but you need a working Mac to do it.
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