As has been said it is ESSENTIAL to FULLY remove all the existing drivers, AMD and Nvidia software do not like each other. At all.
Before installing the shiny new card, go to the Nvidia site and download their latest WHQL drivers, install them when you boot up with the new card and, a has been said, do n't let Windows install anything when you first boot.
Tried to overclock my 6950, did n't work too well either so you're not alone.
Do n't be too remorseful, you've just bought one of the best cards out there