I'd keep Sean Bean as Boromir and Ian McKellen as Gandalf. They were great.
Christopher Lee was a good Saruman, but I don't think he would have been able to do the slippery-Saruman parts. So I think we should maybe take another actor. *sigh* I don't, however, have any better ideas.
I would give
Gina McKee Arwen's role. She looks arwenish and somehow there was something arwenish in her in
Notting Hill.
I'd have Kevin Costner as Ugluk and Viggo Mortensen as Grisnakh.