its always advised to uninstall the old driver
(as roy has stated)....... but not essential
the card going in will run on the same driver as the 8800, if you were just to leave the drivers
, unplug the 8800 & chuck in the GTX285....you would need to reboot twice & the card would pick up the existing driver...
but to answer your question, its best to uninstall & reinstall.