I've heard that even weight of a PSU isn't a good indicator of its quality. Manufacture can cheat that system by adding weights (as simply as increasing the heatsink size, but still have crappy capacitor, transformer, etc.). I think the only way to buy a PSU these days is by researching the brand and the model.
As for the power rating of PSU, 400 watt should be the standard these days. PSU is a part that shouldn't be upgraded every year or so. If you get a decent power PSU of a good brand (400-500 watt), then you won't have to worry about power requirements for some time.