Video Card needs repair?

My 4 1/2 year old 2.33GHz Intel Core 2 Duo laptop starts fine. After a few minutes the fans start running and across my screen appears diagonal lines or vertical squigly lines. My cursor then locks up and I can't do anything with it after that. I can access the computer through target mode, even with the lines on the screen. I've tried accessing the computer from another computer using a brand newly installed version of Leopard, but with the same results. An Apple Genius ran a diagnostic test and said all the hardware is fine. My thought is that the graphics card has gone bad. Is the graphic card separate from the logics board? Can I replace just the graphics card, or do I need to replace the logics board as well. And.. how much is this all going to cost?