I don't know if they (even assuming Robert Rodriguez would be directing all of them) could maintain the energy and style across the entire collection of graphic Novels, but that is exactly what Frank Miller wants to do:
"I'm finishing the script right now for Sin City 2 – I can't wait to get back behind the camera. If I have my way, there's going to be five [films] – but that's if I have my way! Five would take care of all the graphic novels – but there's new material that I'm writing for the second one. There'll probably also be a separate graphic novel that I'll do. It's a Nancy Callahan story that I've been wanting to do."
Sin City 2 has a release date of August 18, 2006.