Just popped the end bit of Matrix Revolutions on, and yeah I think Hugo Weaving would be great.
I can see bits of Eastwood/Dirty Harry in Agent Smith, without it lapsing into Bale/Batman gravely voice.
As for bulking him up - I don't know, it depends on the direction they go with the costume. They deliberately went with Peter Weller for Robocop because he was slight, because someone more muscular would look podgy in the costume.
Dredd varies depending on who draws him. He can be a pinheaded arnie lookalike with one artist, or pretty lanky in the hands of another.