First, I hope this proposed 300 follow-up isn't a sequel, loose or otherwise. Second, Alba's Nancy Callahan is not the title "dame" in Sin City 2. That would actually be Ava Lord, rumored to be played by either Angelina Jolie or Rachel Weisz.
Shut up, you! The media never makes stupid mistakes like that!
I hope they can wrangle up the same cast for pretty much every character. There is probably only one or two that I wouldn't be sad about changing actresses. And they might not even be the same characters so much as grouped together in my mind as the same.
I, too, hope the follow-up isn't related to 300 in story at all. I'm really curious what myth has grabbed his attention.