I had this problem before, and I STRONGLY recommend that you get your card changed. I had this problem with my FX5500 half a year ago, then about a week before, my card was dead suddenly and I can't even turn on my computer.
It is not the problem of drivers, because during the 6 months time, I kept on updating the drivers but the problem still exists, until my card cannot work.....
The final solution is that I went the computer shop that I bought the card, and got my card changed(as my card was still in warranty). I paid for HK$500, and updated to 8600GT, now very good without any problem
