Which Power Supplies Are Compatible?

Earlier today, I picked up a broken PowerMac G5 from a flea market. It's a dual 1.8Ghz model from June 2004. (A1047) The unit wouldn't turn on, so I figured that it had a dead power supply. I confirmed this later, by modifying an ATX (PC) power supply to run the computer. It boots up with the modified ATX PSU, so I'm planning on buying a replacement for the original. The original power supply is a 600W AcBel. I've seen a few different wattage power supplies on eBay, and want to know if I can use a 450W unit (Somewhat cheaper than a 600W) instead. Would this be possible, or would I need to buy a 600W PSU?