I looked to the Google Webmaster Tools to see what keywords are most used on our site. From these words, it doesn't look like our site has a central theme, it's just a bunch of products. We sell discounted goods, in a way similar to overstock.com, so we have many categories. We already have 10,000s of items uploaded with custom titles, descriptions, and images.
Our link structure is set up fine from an SEO standpoint, in that each category has its own path. Our goal is to drive more relevant, organic traffic to the site. What I need to know is would it be better to focus on different high-traffic low competition keywords for each category, or should I try to find keywords that are somehow relevant to all categories on the site?