Writing good SEO content is not enough on its own to reach the top in a Google search. How Google Works shows that there are many different factors involved. One of these is conection between your keywords and domain names.
Here is the URL for the number one result for the search “Britney Spears”:
www.BritneySpears.com
And this 100th result:
uk.reuters.com/article/entertainmentNews/idUKN2233607420080123
Spot the difference? Google can.
Having your major keyword (see What Is The Ideal Amount of Keywords for a Web Site? ) in your domain name is an advantage but it's one you have to balance with other considerations. Such as whether people will be able to remember it. Another consideration is length. Long domain names look like spam sites or cheap, Johnny-come-lately efforts. When looking down the list of results, many searchers will use the domain name as a guide to which link to click.
Except for your home page, the domain name is only part of the URL. Most sites split pages into categories or sub-directories before final, unique document name.
For example:
www.hollyscoop.com/britney-spears/16.aspx
Use your medium keywords for the sub-directory part of your URL as Hollyscoop.com have done here.
Finally, there is the actual document name that forms the last part of the URL as in this example:
www.askmen.com/women/singer/2_britney_spears.html
In most blogging software the title of your post becomes this final part of the URL.
By combining all these together you can make a very powerful URL. For example if our keywords were “Widgets”, “Acme Widget”, and “Left Handed Widget” as our major, medium and minor keywords then this is the sort of URL we are looking for:
www.WidgetsShop.com/Acme_Widget/Where_to_get_left_handed_widget.html
My experience and research with "Britney Spears", "Sex" and "SEO" shows that having keywords in your domain name does have some advantage but the critical point is that your keywords should be in your URL somewhere.
Chris Tregenza runs a variety of web sites including MiceLife, writes free seo articles and a SEO Blog
Better search engine placement through article writing.