Mithril produced a Middle Earth Handbook about 10 years ago that contained a wealth of heraldic detail, based on their research for their LOTR miniatures line. If you can find a copy, buy it. Here's what they say about your specific requests:
Arnor: Round shield, seven 6-pointed stars evenly spaced around the circumference starting at the top, with a winged helm in the center of the shield. No notes on colors, but based on Cardolan (see below) I'd go with purple for the background, silver (white) stars and helm, and silver (white) or golden (yellow) wings on the helm.
Arthedain: Same round shield with seven stars as Arnor, but with no central design. Same colors.
Cardolan: Same round shield with seven stars as Arnor, with a "hill" emblem in the center of the shield (looks like a pyramid in the drawing). Mithril notes a purple background on the shield, but no notes on the color of the stars or hill. Mithril does note that their armor was "brightly polished or silver plated", so I'd go with silver (or white) stars. Probably a gray hill.
Rhudaur: Mithril has no notes on shields for Rhudaur, but they do say, "The Knights of Rhudaur were grim men, and thought of uniformity disappeared early in their struggles. They often wore their hair long and wore fur capes and cloaks for warmth." If you want to use a shield design similar to that of Arnor, you could use the round design with seven stars, and for the central emblem use a stylized Last Bridge or watchtower. See this excellent discussion of Rhudaur for my reasoning: link
Osgiliath: Same round shield with seven stars as Arnor, with a "tree" emblem in the center of the shield (classic bushy green top, with a trunk that splits into a triple root at the bottom). Same colors as Arnor, with a green and brown tree. Mithril notes that this design was also later used in Minas Tirith.
Mithril says nothing about banners, but any variations on these shield designs would look good.
If you don't want to use purple on all the shields, you can vary the color by country: purple for Cardolan, royal blue for Arnor, purple-blue for Arthedain, blue-gray for Rhudaur, navy blue for Osgiliath.
Mithril also describes the armor and helmets, so if you want that information, too, or if you want to know what Mithril says about any other races or nations, let me know.
Jeff Moore
jpattern@aol.com