When you buy a PSU (or choose the one they install for you), look at the side of the unit. There is usually a label with a list of the power the PSU can safely provide to your system. In this list the important number (that actually tells you whether the PSU is good or bad quality) is the one under the 12V column. This number should be at least 25-30A. Otherwise the PSU wont be enough to handle new display cards or fast HDDs etc.

What many people (even legitimate PC assemblers) do is try to pass off crappy PSUs to their customers by telling them that it is 400w or 500w or whatever, but in reality it is usually some no brand unit that has a very low 12V value. Since most people dont pay much attention to what PSU is going in their system, they are able to get away with it quite often too.