This is a recall issue that Apple is still covering. Please review the following questions to get your machine repaired for free: http://www.ifixit.com/Answers/View/349/Vertical+lines+appearing+on+displayReview
Review these urls to be really prepared before contacting Apple:
Apple iMac G5/1.8 17-Inch (ALS) Specs
Identifiers: Ambient Light Sensor - M9843LL/A - PowerMac8,2 - N/A - 2055
This is a recall issue that Apple is still covering. Please review the following questions to get your machine repaired for free: http://www.ifixit.com/Answers/View/349/Vertical+lines+appearing+on+displayReview
Review these urls to be really prepared before contacting Apple:
https://discussions.apple.com/message/13112006?messageID=13112006
https://discussions.apple.com/thread/1146388?start=345&tstart=0
I have one already repaired, it was a Core Duo 1.83 and I installed a 2.16 GHz C2D and it bench marks the same as a 2008 20" 2.66 GHz iMac