and it still did not work, then that points to a logic board problem.
The graphics chip is located on the board and is not removable.
Bad chip = Bad board.
Sometimes it works, sometimes it doesn't.
So I have a Macbook pro 17" (non-unibody) and one day I turned it on and heard the opening chime, but the screen did not turn on.. and the computer itself had turned on. After visiting the mac store, i was told that it was a problem with my logic board and that soon it wont turn on at all. Then a few days later I turned it on and all was working! the screen was on and all... its been a month now and it turns on normally now. Im not using it anymore though, Im using a spare laptop I have around..
So what could be the problem? I dont see how this could be a logic board issue.
Also, I changed the screen completely at a repair store and the new screen also would not work. Im confused!