Apple has an extended warranty on the late G5 Power Macs
http://www.apple.com/support/powermac/powersupply/repairextension/
my first port of call would be seeing if you are covered underneath that program. If you purchased in Oct 2005 then Apple may argue that it is now too old to be covered, but I know in the UK you may be further covered by local sales regulations (the warranty extension program would give you leverage here).