key words

is it best to try to cover all possible keywords n a website or better to focus on the more important key words for a site?

lets say that your keywords are 'nigritude-ultramarine'.
firstly....you need to think in terms of 'pages' as well as 'site'.
google works at the page level.
next...it depends a great deal on your pagerank situation.
if you are just starting out on your nigritude-ultramarine site.
then you might want to focus your pagerank [and all of your nigritude-ultramarine keywords] on the index page until you can build up pagerank on the rest of your nigritude-ultramarine site pages. after you establish your site and it gets large enough [deep] and has substantial pagerank on the deeper pages...then you can make specific pages target more specific keywords [niche words] like 'nigritude ultramarine parts' or 'nigritude ultramarine equipment'.

its also best for newer sites to target more 'niche' keywords than to go for the higher competitive words until the site establishes more pagerank and site depth.

Your most important keyword/keyphrases on you index/home page. Try to keep it to a max of three keywords/keyphrases. Then associate your internals with the balance of your kw list. Make sure they are relevant to the page and associate no more than three to any given page. Anchor link to your internals from the index/home page. Ensure also that you have a site map link on your home page. At least that is what I do.

When you site is new, pick just a couple of very easy keywords for each page of your site and get ranked for them. This will get you more traffic than being #357 for a popular term. It will also get a small flow of money through sales or affiliate programs.

Then as your site gets more pages and inbound links, add a second very easy keyword to each page of your site and that will double your traffic. Don't think that this will take a lot of time editing. If you design your site with this strategy in mind you can make these revisions quickly with "find / replace" tools.

After you are smokin the competition on these easy terms, then it is time to keep them in the text and headers of your site but pick a few more difficult fights with your title tags and off page SEO.

Keep at this and keep building pages and getting more links....... soon you will have a site that vacuums traffic for thousands of terms.

specific keywords or keyphrases are always better than generic keywords in terms of rankings and conversion.