Power supply ratings are completely imaginary and invented by the manufacturer. How many amps you rate your PS at depends on how hot you are OK with it getting and how short a life you will accept.

Manufacturers of cheap power supplies are perfectly happy with high temperature and short life, lots of ripple - and blowing your motherboard when they fail.

Antec is a reasonably reputable company, so their ratings are pretty conservative. I've been happy with their recent products which I've gone to since my previous supplier only does container loads now.

I used an unfamiliar 400W power supply I got with a cabinet I was evaluating in an older machine that had run 5 years on a 250W from my former supplier. The bassard went out in 4 months and took everything but the hard disk with it. Only time I've ever had a failed supply take out anything else.