The PSU is sufficient, but those eXtremePower 650W units are based on an older 500W Seventeam platform, and certainly not capable of putting out 650W - if you're going to get it make sure it's priced accordingly.
With regards to Vista, you'll likely be asked to reactivate, and you may need to provide Microsoft with your activation details over the phone. I've never had any problems doing it, though.