There is a little more to the story here. What you really need is the voltage and current requirements of a mendel and different components. Wattage ratings of pc power supplies are not as accurate as you would think. Also consider the voltage drop on the 12 v rails.
You will need about 500ma at 5v, 7.5-10amps for 12-20v for the steppers and drivers circuits as well as single extruder, and about 12-15amps 12v for the heated bed. This is of course for 1 extruder. Add about 4amps per extruder as a safe precaution. If you ever over current, the supply will turn off on you, if it is a good one as a safety measure for you and for the device.
If you are looking at pc supplies look at the voltages and current supplied for each rail and add about a 20% buffer for your amperage requirements.
Hope this helps, IMHO.