I started this thread a few months ago...might give some extra insight.
http://forum.calgarypuck.com/showthread.php?t=62851
IMO give them no more, no less than 2 weeks notice. The way I think about it is would they give you anymore than that if they had to lay you off? If you give 4 weeks, and they send you home they will only pay you for 2 weeks, so now you are out 2 weeks pay.
Be professional about it and like others have said don't burn any bridges, but also be selfish and think of yourself, not this company who probably wouldn't bat an eye at laying you off if they had too.