How To Change Dell Power Supply To ATX Power Supply?

I have a Dell Optiplex GX400 and would like to know what type of power supply it has and if it has the special dell one how would you go about changing it to a ATX power supply? This all links to my previous question about changing the case of my Dell Optiplex GX400 located here: http://answers.yahoo.com/question/index;_ylt=AuSP_ZxSaLO8SZZu44zgi4_sy6IX;_ylv=3?qid=20091116104221AAooUAY