The domain name itself carries little weight in the overall ranking criteria, however... there is a reason that a keyword-rich domain name can help you to (indirectly) improve your search engine ranking.
The reason is because of the way that people will link to you...
Currently, the biggest ranking factor (for Google results) is "off the page" or your inbound links - the quality of those links, the relevancy of those links, and how your pages are linked to.
If someone really likes your website, and decides to link to you... they are most likely to link to you with the Anchor Text (or Link Text) of your domain name. For this site they generally link using text such as "ClickNewz", "Click Newz" or "Lynn Terry".
But if you want people to link to you using your preferred Link Text (to help with your rankings), you have to specifically ask them. As we all know, link exchange requests are getting harder and harder - people simply ignore them for the most part. So your best links are going to come from those who just link to you naturally. By choice.
This is where a keyword rich domain can be helpful!
Lets say that your domain name is "lowerbloodpressure.com" - people will naturally link to you using words like "lower blood pressure" (which contains your keyword phrase). People do that because it's the 'title' of your site, and its easy (ie they dont have to think).
My own domain name is a good (bad) example of this: SelfStartersWeeklyTips. Many people link to it as Self-Starters Weekly Tips or SSWT. My primary keyword phrase is "Learn Internet Marketing" so I have to really work hard to get inbound links using that anchor text - and ask for it specifically. Ideally, links on other people's web pages that are pointing to my site would look like this:
The anchor text (or link text) used in linking to your pages is directly related to which terms your pages will rank well for.
So if you name your site - and choose your domain name - around a good solid keyword phrase, you'll be more likely to get a higher number of your inbound links with keyword-rich anchor text that helps your main page to rank better in the major search engines. 😉