I personally would probably prefer unknown actors just because I love the characters so much and it would be hard for me to now associate them with an already recognizable face. I think Reese Witherspoon might be a fun choice for Stephanie. She can pull off sexy/funny. Kate Hudson or Alyssa Milano as well. The rest I couldn't even begin to guess.
I can see Sandra Bullock as Stephanie.......a lot actually. She does the unconventional girl thing very well. I love these books, by the way. I've sat there laughing out loud, looking like a lunatic, while reading these.