having a blog aswell as your main site will help in seo very much! AND if you do a few tweaks to that blog, ESPECIALLY on a new website, you should get very good ranking within a day or two...so YES
Blogs are GREAT for your site also join some of the free blog sites and post your blogs there also. Main thing is good up to date information. One thing I do is write several blogs at a time then save them in a word doc then copy and paste them as needed during the week so I always have new info when I don't have time to write a post on a particular day I just go to my saved word doc..
Blogs will help SEO only if you blog regularly with fresh content that are keywords targeted. Also set up standard ping so that search engines would automatically know each time you post new content on the blog.
To promote your site you can use blog in two ways: 1)within your website like installing blog from wordpress.org on your domain. This will help to provide your website with a new content. Any updation in the blog will be counted as updation of your website. This way you can provide fresh content on topic to SE and users. 2)using external blogging tools like www.blogger.com, wordpress.com or livejournal.com. This way you can get link to your website within the content and if your content is good and powerful people might link back to your post. This way you can get links to your website. Use the above two blogging method to get the benefits to your website...