http://www.gamefaqs.com/computer/doswin/game/3365.html
On this page is the technical tweeking faq.
Following all the suggestions, I've more than doubled my framerate even in big exterior cells like balmora.
Don't forget to turn the music off, and defrag your hard drive.
Oh yes, for those of you who are less technically savvy, when you turn on quadratic lighting, you turn OFF linear lighting...
"UseLinear=0"