Hi Vanessa,
Articles can help your
SEO if they get indexed themselves for your targeted search terms (keywords) or indexed for terms that are close to your targets. You then need to link back to the relevant pages on your site using good anchor text in those articles.
You'll find plenty of folk saying spread your articles far and wide but in practice this is usually misguided as they will not all get indexed if they are the same article.
If you are going to try article sites for backlinks then I'd recommend just the top few. Ezinearticles, Goarticles, Wordpress.com, Blogger.com and you could try Squidoo and/or Hubpages too if you want to be a regular contributor in those social media outlets.
Try any or all of them but note it is generally worth the effort to re-write the articles to make them unique and improve your chances of getting all of them indexed.
The thing is, it is usually not enough just to post the articles, you also need to get those articles noticed, at least a little, so you have to do a little promotion of those articles too! Tweet them, bookmark them, see if you can get links from blog comments, whatever you can.
There are numerous tactics you can try to get some juice out of article links. Another is to have the main 'pillar' article on your own site, then a unique but cut down version on Ezine (or Squidoo/GoArticles) then a further unique cut down version on Wordpress or Blogger with the last one linking to both the 'pillar' article on your site and the version on Ezine. There are several variations on that tactic but I won't get bogged down on them here.
Want to get the best use of articles IMO? Instead of using them on article sites, try to get them published on good related blogs or other related sites that welcome guest posts!
Give the site owner a unique version of the article to use as content on his site. The payoff for you is your choice of anchor text links back to your own site. Don't over do the linkback requests, 2 or 3 links back to different pages on your site is usually acceptable to the publisher and plenty of juice for you.
My 3 cents worth...