The overtemp light is coming on...

This model of PowerMac G5 is most likely watercooled. In these models, the coolant tends to leak, and spill onto the power supply, and motherboard. The cooling systems also can fail altogether (The pumps can just die). I recommend that you stop using it, and check inside the case for any spilled coolant, or corrosion (Which is a result of spilled coolant) Unfortionatly, this isn't a problem that you can fix yourself, as swapping CPUs/Cooling units will cause the computer to (In most cases) run the fans erraticly. If cooling leakage is the problem, you should take it to an Apple Store. For a while, they were replacing faulty leaking G5's with Mac Pros, but I'm not sure whether they still do that. For more information, look at the first answer of this question.