Same thing with my Powerbook G4. But I knew it was gonna happen when I heard the hard drive start buzzing like a bumble bee. I rebooted....Gray Apple. So to fix it, I bought a Dualcore 2.66gHz Mac Pro with the ATI 1900xt graphics card and 4 gigs (all of them not from Mac, as they are more expensive than organ transplants).
To test its power, I installed Civilizations 4 with all the graphical meltdown turned on. Runs like a champ. Now if I can just stop playing the damn game, I might be able to do some video editing.