Someone was wondering if Leopard would work with aftermarket CPUs, like those from PowerLogix. Here's the answer from an OWC (http://www.macsales.com/) email: "Note - Leopard requires that you have a G4/867MHz Processor or faster in your Mac. So what about processor upgraded machines? You are GOLDEN (at least so tested with our lines) :). Any PowerMac G4 model upgraded with an OWC, PowerLogix, or NewerTech model upgrade G4/900MHz or Faster meets the processor requirement for Leopard. Dropped a NewerTech G4/2.0GHz into your Sawtooth 350MHz - the cat will be very happy and your investment is safe. Also requires - DVD Drive, Built in FireWire (All G4s & later have this), and 512MB min of memory." -- Rich