Drop the bows on the twins.
Lose Arvedui, replace him with Arathorn. Three 3 attack heroes will be lethal. Or if you're taking Arvey for his stand-fast, just remember: a larger stand fast is no good if your hero is dead. At 750pts+ I'd always make an effort to include Halbarad with his banner of Happiness and Inspiration(tm) and an Arnor Captain. Sure the banner is expensive, but if anything its actually undervalued - double the price of two banners, but double the range *and* auto-pass courage. That is absolutely invaluable for Arnorians. At 1000 replace the cap with Arathorn.
I've taken Arnor to two out of three tourneys this year, and as a relative gaming beginner and taking non-competitive lists I think I can happily say I've got to a decent standing with them. But my current project is looking at how to make a competitive list using Arnor as a main basis, and I think I've stumbled upon the real ultimate list, but you'll have to wait and see to find out
*Edit: Addtional note about Arvedui vs Halbarad. Larger stand fast seems appealing, sure, but running away is actually the least of an Arnorian's courage worries. The real killer is Terror causing models and not being able to charge. Up against a wraith? Let me tell you from experience, 40 something troops at courage 1 is not nice... the banner, however, negates that. Besides, if you're broken you've probably already lost, and a smart opponent would tie up Arvey before he can test for courage/stand fast. He's a beautiful model, but apart from fluff there's really no need or incentive to take him
