can faulty graphics card hinder iMac startup?

Hi,

My early 2008 24" iMac, which I recently equipped with an SSD (running El Cap) to replace the original HD, is not starting up. The startup process gets stuck on the white apple screen, about a 1/3 up the progress bar. The screen goes black, then it restarts again a couple of times and finally I get the prohibitory sign.

I've tried and failed: recovery mode and safe mode, and reinstalling the OS from a USB drive.

Also tried opening it in target mode on another computer - but the other computer is even older and running OS 10.6.8 and that may be the reason it couldn't mount the SSD.

Finally I decide to take it to a lab. This is a decision I tried to avoid because where I live there aren't many Mac users and I'm never sure the technicians have enough experience to know what they're doing and I doubt they have the official Apple diagnostic tools.

The technician said that the SSD is fine and the problem is cause by the graphics card. The repair would be too expensive to be worthwhile.

My question is if this diagnosis makes sense, since the screen displayed the appropriate color and graphics for the different things I've tried (till it got stuck).

Thanks!