Yes - ATX power supplies get very upset if run with no load (don't ask how I found out
)
Is this bench PSU for a computer, or something else?
I wanted a 12v PSU to run some kit up in the Norfolk cottage. A nice 12v 15amp one with a transformer, bridge rectifier, and transistors to hold the output voltage stable was £100.
I got a switch-mode one, 13.8 volts max 20 amps for £20. It will need some added protection circuitry before I use it to power anything expensive, but at that price I don't mind.