$600? That's a lot of dough. You can upgrade the board, cpu, ram, and video card to PCI-e, then sell off the current parts. 6800GT is near the top of the AGP spectrum so you won't be going far. I think the only faster cards are some X800 and X850 variants, the 6800 Ultra, and the 7800GS.
Of course, you can always choose to save the dough by spending moderately, say ~$120 on an X800 variant to get similar performance to what you're accustomed to.
In any case, chances are you can recover some of your money by selling the dead video card on ebay. Some people may want to try their hand on fixing the part.