Dont replace with a atx power supply bad idea really bad idea.

had one tried the tutorial the power supply worked good for 3 days then nothing the machine became unstable and then fried the cpu apple had rigged power supply's for a reason and by the way your graphics card is causing your issue if you have a mac graphics card that requires you to have a 300 volt power supply to be able to run it the g4 quicksilver is a 300 watt power supply which will cause the power supply to fail do to not enough power