The newest operating system for this iMac is macOS Sonoma.
The newest operating system for this iMac is macOS Sonoma.
You still have the same system but with more RAM. It should handle newer operating systems better. Installing Linux might be possible now, though you faced issues with Lubuntu 32-bit; trying a different distribution could help.
It's been emphasized multiple times that they won't support anything newer than 10.7.5 Lion. Even upgrading to 10.9 Mavericks would be challenging and wouldn't deliver good performance. If you require a fresh operating system, Linux is the optimal choice.
I've tested several variations of Linux (arch32, CentOS, Lubuntu) but none functioned properly. The installer rEFIt reported: "The firmware refused to boot." Although Rocks! OS worked with CentOS 7, I didn’t want to install it there. What’s the best approach now?
Ignore the issue with rocks. It functions somewhat. Edit 2: It appears to display a fixed image.