Should I Post to Article Directories?

Without any doubt article submission is useful. but submitting the same article can drive some problems. so always spin the articles before you submit. don't submit too much articles a day.
when Google will index the articles your SERP will be increased. how much? it depends on the competition of the keywords.
you should never copy the same article in the main site.

Everyone is going to give you slightly different approaches to article marketing.

The question is - are you doing this for backlinks or to drive quality traffic to your site? You said it's to drive traffic, so that's what I'm going to assume. In this case.

1. Write a high quality article that grabs people's attention, solves part of a problem, and then sends them to your target page - one that solves or sells something that solves the rest of the problem.
2. Post the article on your website first. Wait for it to get index.
3. Post it to the most relevant, high traffic directory first. Usually that's EzineArticles, but it depends on your niche.
4. You can then submit it to other directories. I wouldn't bother with anything that just has junk on it. That won't get you any traffic and just sucks up your time, unless you are outsourcing it.
5. Build links to both the article on your site and the ones on the top directories. This will give them a little extra boost and potentially rank them within the directories themselves and on Google. However, you will only see one instance at most of your article in top rankings.

I don't like spinning articles at all. It takes a long time to set up one that will produce anything vaguely readable. Those types of articles are more for building backlinks than driving traffic. They're not meant to be read. That said, there are certainly plenty of people who do it.

Good luck!

Sharyn