I have the same card, go to
http://www.nvidia.com/page/home.html, click download drivers, find the linux ones, put it somewhere and execute the program ("./Nvidia-foo"). If you afterwards run nvidia-xconf or something similar your xorg.conf is automatically adjusted. Alternatively you can edit xorg.conf by hand and change nv into nvidia. For compositing support add:
Section "Extensions"
Option "Composite" "Enable"
EndSection
to the very end of the file.
Good luck.