amd is recommending a psu not just the 12v rail and they have to rate higher than actually needed to make up for all the crappy psu's that can't put out their rated wattage. Truth is a quality 450W unit will run any single gpu system.
But samuelspark is right, the XFX is easily better. And don't see why you couldn't use the VP450P.
Shows a system using an OC'd i7-920 and 6870 drawing 306W at the wall under Furmark. Furmark is a torture test, so power draw is usually much higher than in games.
Combined this with the fact those numbers were taken at the wall, means you have to take 10-20% off that number to get an idea what the PSU was actually delivering to the components.
So if there system was only drawing 306W it is unlikely that your system will draw more unless you are running torture testing programs. So in reality you only need a high quality PSU with the necessary connectors that can deliver at least 300W continuously.