Using way oversized PSUs is just a waste of money - and electricity. A PC type PSU runs fairly well at 50% load and up; below that it will burn a very high portion of the input wattage and heat the room. A 500W PSU with 150W load runs with very poor efficiency.
Try Amy's standard PSU and measure voltage levels with full load. If the levels are not below 95% you don't have any problem here at all. If you really need replacement, try a good quality model with low wattage (250W): you won't be wasting energy, have a lower noise level and there will be no problems with peaks that a cheap, oversized PSU might generate.
