If you do turn out to buy a new PSU, I've been told the rule of thumb is that PSUs operate best at around 50% of their rated capacity. So if your system uses 632 watts, you would want a PSU in the 1200w range. An older power supply rated for 650 at peak is likely not going to be able to keep up with a modern system trying to draw ~630

https://en.wikipedia.org/wiki/80_Plus

The wikipedia article on the 80 plus certification program seems to back that up.