I've got a friend with a G4 733Mhz Quicksilver Power Mac. She had a stick of 128Mb and 256Mb RAM in it and fitted another stick of 512Mb RAM. When she powered up, OS X couldn't recognise the new 512Mb. She turned it off, took out the new stick and turned it on again. It now only whirrs for about 5 seconds before it powers down. So far she's tried: pressing the PMU button taking out the battery, leaving it, putting it back and pressing the PMU button Every time it just lasts 5 seconds before turning off. I'm going to try and fix it in a few hours, but does anyone have any suggestions?