I suspect my G5 PowerMac wot won't start up and has weird-coloured squiggly lines on the monitor has a dodgy or poorly-seated video card.
While it won't start by itself, it does appear on another Mac as an ext HD when booted into Target Disk mode.
I rebooted the second Mac and chose the G5 as the startup drive. Everything proceeded nicely, just at the speed of a G4 PowerBook with 1.25 GB of RAM rather than of a G5 tower with 13 GB of RAM.
Later today I'll have a play around with the video card.