Duplicate content on your site, that is the same as others will not help you with SEO, SEM.
In fact it can harm your results in the serps the more sites with the same content, most likely the top five search eng's will not count the dupe content as a plus for you.
Original content that is relevant content and written for people not keyword mashing for SEO is the best way to help with SEO / SEM.
So, not to put down pre written articles. However taking the time and writing your own in your own words will help your site with SEO / SEM. But only if the rest of your site is up to par.
I suggest you visit http://www.SEOMOZ.org/ and read as much as you can from the blog section from top SEO professionals.
Also you can do a basic site SEO checkup at http://www.seositecheckup.com/
I hope my response has helped you a little in your decision as what is best for your site.
-Ken