Not a lot of actresses come to mind for Julia except Gweneth Paltrow. I think she'd be perfect for the role.
And about Yoko Kanno, I know she is the heart and soul of the series, but you guys have to remeraber, this isnt Watanabe's Bebop. This is Hollywood's interpertation of the series, and as sad as I am to admit it, Yoko will probably not have any music in this film. Warner Bros will probably try to market it as a 'Star Wars/Star Trek WITH ATTITUDE' action bonanza, meaning lots of rock/techno music, with one or two poignant/emotional songs/music, probably for Spike's memories of Julia.
Or, they could try to adapt the manga of Bebop, which, to my knowledge, came before the anime, and did not feature Julia or Vicious.