What type of ID's are needed for US Green card holders travelling to Canada by car?
Is passport required or would green card be sufficient?

Take The Passport!

Actually according to the Canadian Consulate in the US the green card is enough. If you do have your passport I'd bring it, though I'm assiming the point of the question was that you don't have it right now for some reason.

If you want to be sure, call them, the phone numbers are in the second link

http://www.amcits.com/enter_canada.asp
http://www.amcits.com/phone.asp