Choosing the right resistor for a LED circuit
for a LED lamp you typically want to run the LEDS at near rated  power, to do that you need to control the LED current and for that you need a higher voltage than the LEDs and some form of current limiting (like a resistor).

Use the 5V output from your ATX supply and use a 1.6V / 0.600 = 2.7 ohm  resistor in series with each LED. use 1W or larger resistors for each LED.