G5 imac terminal display problem?

Hello
I think my 20" G5 iMac ( the model before the integrated iSight) may have what amounts to a terminal graphic card problem. It was working perfectly until suddenly an odd red pattern made up of what looks oddly like thousands of tiny i-ching symbols overlayed over the actual desktop. I have tried rebooting from the CD which suggests it isn't a software issue, and since I understand the graphics card is part of the logic board, it's probably not worth replacing it financially. I am hoping (but sadly not expecting) that someone might be able to prove me wrong, or else it's off to the cheap PC shop for me :'-(.
Thanks