Should I keep the 5 star rating control for rating articles, and change the voting on items to thumbs up/down? Or should I change both the voting on items and voting on articles to thumbs up/down?
Both articles and items should have the up/down vote options, rather than the 5 star, IMO.
There may be value in the determination of consumer-side article rank by a (weighted) average that encompasses both the net rank of items posted to the article + the rank of the article itself.
I'm sure you've already thought about the item/article interaction in terms of effect on overall article ranking. UI/X-side, the effect (of ranking an item within an article) is unclear now, and I am fine with that as an early user.
Moving forward, developing a ranking formula that weighs article rank against net rank of all of the article's items will be a usage-informed improvement that provides great value to users.
That ranking formula, as well as making the result of a "ranking" interaction obvious to the user, will help the understanding of how such interactions work.
With a social answering/ranking/voting application, creation of, and/or tweaks to, the formula that determines the effect of interactions will have a huge influence in shaping the culture of the individuals that interact with the application.
Judging from the concept of "decision fatigue" outlined in that NY Times article the other day * , you're better off with thumbs up/down since it's less work to decide that than to have to choose 2 or 3 or 4 stars for an OK article.
Think about it from a game theory perspective. If the rating of something is higher than you would like it to be, why would you vote more than a 1? If the rating is lower, why would you vote less than a 5?
Some people may express their honest preference, but a major motivation for people to interact with something like this is to change the average, and voting at the extremes maximizes their power.