We are a small company selling copier equipment online. We have limited time and resources. We just finished rewriting our URLs to be more SEO friendly. We want to continue our SEO efforts, but want to make sure we use our resources wisely.

So, which of the following would have the biggest, most immediate SEO impact: better incorporating keywords into our product pages, writing original content, blog writing/guest submission, social bookmarking, SEO friendly filenames for images and use of alt attribute tag, inserting microdata (especially on product pages), social media, inserting meta descriptions?

Is there another SEO tactic that we should be considering?