Ha ha, don't worry Source, I wasn't gonna do anything as rash as that!! I'm happily running Leopard on my newer machine but the G5 will stay strictly Tiger only. My main concern was software version conflicts, but the other guys have re-assured me there. [phew!]
Have ordered an SSD as a SCSI drive is starting to get a bit shouty now, so fingers crossed we'll all be good from here...