The rated wattage isn't everything about a power supply.
Firstly, that may not represent the real power capabilities for a few reasons.
- It could be rated at peak as apposed to continuous. Peak rating can be 100W more and usually useless for the consumer.
- It could be rated at a lower ambient temperature. Due to de-rating curves PSUs can generally deliver higher amounts at lower temperatures. Typical temps in your PC are 40 - 50C.
- The 12V rail is the one that matters, and account for ~90% of your power consumption. The 12V power rating is more relevant than the overall wattage.
You should be looking the PSU's quality as well. How stable are its voltage delivery, how low is its inrush current, how well does it deal with EMI, how long does it last when AC power is lost, how well crafted it is and quality components used on a good design, how quiet and efficient it is.
For compatibility concerns you should also account it's physical size, connector amount and length, maybe modularity.
We need to know the PSUs model number.
A high quality 650W PSU will be fine.