THe silverstone psu might be good, but not that good.
Even if it hits 85% efficiency on a 450watt psu it will only produce: 382 Watts.
That's not how it works. A 450W psu can deliver 450W output. In producing that, some of the input was lost to heat. This means that in order to deliver 450W, draw from the wall will be 513W (110V) or 504W (220V).