I've got a 9800gx2 had it for about 2-3 years now died on me yesterday it seems they are somewhat prone to heat causing faulty connections.
I stripped it and baked the PCB's in the oven at 200 Degrees for 10 mins new paste and a good clean and she's fully fuctional again.
Not something for the faint hearted and don't even think about it if the cards under warranty.
If it's out make sure you get the card tested in another machine to establish it's dead if so baking method might bring a new lease of life back to it.
Worked for me and many many many others.
But its a definate last try nothing to loose thing.