iMac 2008 vertical lines I have an issue with my iMac 2008 (8.1) (vertical lines). 

And I don't know where is the problem GPU/LCD. 

Someone said I must buy mini DVI to test if the the screen is defective using a external screen?

How do I test if my LCD screen or GPU is defective.
The other commenters are correct. With an iMac you need to try an external monitor to determine if it is the screen or the GPU.

An Apple Genius appointment would cost you nothing and they could test it for you if you don't have access to another monitor and/or the right cables.