Review Stymiee's Sitepoint SEO FAQ. By the question you asked I think you need to do a little more research into on-page optimization; no, you don't optimize content. You write quality rich content for visitors NOT search engines, keywords will naturally appear within the content itself.
first of all choose your keywords and then focus on content and don't use too many keywords in content alwayse use around 10 to 15 keywords in content of one pages and after that focus on linking. cheers
first you have to write content that attracts customers & improves conversion rates, and keyword density gets imporve naturally.
I can't imagine a scenario where you would have to choose - your keywords are a shortened version of your content - your content contains your keywords - where's the conflict?