After upgrading kernel package, on Ubuntu 8.04 I always had to reinstall NVidia's Driver.
Fortunately nvidia provides you with an easy way to do it, and this way you make sure you always have the latest one, without having to hunt for the driver at Nvidia's site (which means a LOT! Nvidia's site browsability really sucks).
Just type the command
$ sudo nvidia-installer --update
and it will do all the work.