is the memory controller in the chipset?

if it is, you might need to re-apply heatsink grease. Also check obstructions in airflow, especially cables. Been a while since I opened an iMac G5 but 80c is what the temp of a lot of things run at in a machine, especially Laptops. Also, the iMac G5 isn't the most well-cooled thing Apple made. first things first, pop off the back and check fans/ obstructions.