Yes you can pull 24v out of an ATX power supply. Dunno how mod crazy you are, but you would need to connect the + line of the fan to a 12+ line on your PSU, and the - line of the fan to the 12- line on your ATX mobo connector (NOT THE GROUND! THE NEGATIVE TWELVE LINE). Look up a diagram of the ATX connector to find the 12- line. I've tried it myself with both some regular 12v 80mm's and a voltage tester. 24 volts HTH