That is a voltage sensor error probably on the power section of the logic board. It should not cause a video defect.
How soon do I need to contact the Apple Store in my city?
According to the MacTracker database, that old an iMac is considered "vintage" by Apple and therefore they will no longer work on it. I had a 2007 MacBook Pro fail earlier this year and was told the same thing. You would have to seek a Mac-savvy, third-party service provider.
The stripe is likely a problem somewhere in the video hardware and those are seldom picked up by Apple Hardware Test. To see if the problem is on the logic board (not worth fixing) or confined the the display itself (maybe worth fixing), attach an external monitor (an adaptor may be required). If the stripe does NOT appear on the external monitor, its in the video cabling, the inverter, or the display itself. If the stripe shows on the external, the video hardware on the logic board is failing.
Neither is a cheap repair--probably more than the approx US$300-450 value of first-gen Intel iMacs†---and your firm may consider retiring that unit after backing up the hard drive contents.
†--estimated from values on www.everymac.com