Naomi Watts is going to be playing Narcissa Malfoy (Draco's Mom) and Stuart Townsend and Joseph Fiennes (Ralph Fiennes Brother) have also been cast but it hasn't been anounced who they are playing ( I think Stuart will be playing Bill Weasley and Joseph is going to be Tom Riddle Sr, or Morfin, or Fenrir Greyback)
Source:
http://www.mugglenet.com/index.php (scroll down a little)