Power supplies can vary as much as 50 watts from their "rated" output, depending on how they were rated and how honest the manufacturer is. In addition, many cheap power suplies do not include thermal or electrical overload protection, and use smaller or fewer capacitors, resulting in increase power spikes. They can also be improperly shielded, causing weird crashes from RFI.
The worst major brand that I have dealt with is Antec. Most other major brands are good. The quality of lesser known brands varies widely from excellent to extrememly poor. You can actually feel a weight difference between most good power supplies and most cheap ones, with the cheap ones weighing up to 1/2 as much!
I'm so tired of cookies I'd settle for spam!