If you go to Dell's site and search on the word "video" and "rotation" you'll get a listing of 21 Radeon and nVidia items under Downloads.
BTW, I have 128MB DDR ATI Radeon 9800, and have had no problems since installing the new driver. I assigned hotkeys to the rotation feature so I can pivot the display without having to go into display properties. Awesome feature -- fantastic in the word processor, but some things (e.g. web browsing) are slower in the portrait mode.