Obviously Domain Names play an important role in SEO. Any domain name with .edu .gov etc., will hold a higher page rank then it's .com counterparts. Additionally .com or .org is weighted higher than any other domain name extension.
When purchasing a domain and building a web site to be SEO Friendly, it is imperative that you have your main keyword in your domain name, page title, h1, etc. You should attempt to drill down your web site in this way so that the main keyword is prominent over and over again - then support this with some good unique content that has a good kwd or keyword density.
Also you should always purchase your domain for 3-5 years or more, Google believes this shows that you are not a fly by night operation.
There is literally a 1000 little things like this that influence your overall page ranking. So every little detail from the beginning should be considered in order to get a decent page ranking.