The better quality the PSU, the more accurate it will distribute. If you buy some no-name brand, it can be quite faulty... And doesn't last very long (and I am talking about normal conditions).
If you really want to get the maximun potential (power wise) then u are going to need something that gets more than just the job done!
There are many other factors in a PSU besides having enough watts, etc. Proper distribution is very important at all times.