PSU’s are generally universally compatible. The connectors not so much ;). as long as the output values are about 5%-10% of each other in voltage you should be ok. The system shouldn’t pull more amps than it needs unless there is a different problem. The rule of thumb is generally go higher on wattage when replacing a PSU.
As for the RAM the heat has to do with the diodes and resisters sizes. newer ones are smaller so use less power to do the same thing as the distance between layers is less so require less power to traverse it. This being said heat is what always kills these types of components. also the hotter they get the more easily the electricity can flow between layers resulting in errors and potentially damage.
additionally I remember working on a apple 2(i think) back in 1996 and the heat the RAM gave off was horrible and they had to design the case so it had more airflow after that model because of them failing. Those things could(and did) heat up my classroom in the winter were it was really cold!