YES you should have a sitemap! But no, it likely doesn't affect your rankings a whole lot (an HTML sitemap might pass some link juice).
Create both an XML and an HTML sitemap.
HTML SITEMAP
1. To help you create one use the Xenu Link Sleuth:
http://home.snafu.de/tilman/xenulink.html#Download
2. Link to the Sitemap in your footer.
3. Add a meta robots tag for 'noindex, follow'
- sitemap pages are thin and shouldnt be indexed. however, you want the spider to find and crawl the page to find new links when they are added.
4. this page is for users so make it easy to use, a list of links kills me. i prefer the 'tree' layout design for HTML sitemaps, but since its not indexed go crazy and do something epic.
5. If your site updates dynamically (UGC, blog, etc..) then use the sitemap page to link to 'areas' of the website and NOT specific posts, items, etc..
6. If it is a static or manually updated site then link off to the most important content that you want users to find easily
7. Make sure the premo pages are listed first, that is the ones that are most important to you. a spider may not crawl all the way down.
XML SITEMAP
1. To help you create one by hand use the Xenu Link Sleuth:
http://home.snafu.de/tilman/xenulink.html#Download
2. To make a manual XML sitemap review the Sitemaps.org Protocol:
http://www.sitemaps.org/protocol.php
3. To have one autogenerated for you use a tool like this one:
http://www.xml-sitemaps.com/
SUBMITTING SITEMAPS
1. Make a Google Webmaster Tools Account here: https://www.google.com/webmasters/
2. Claim your domain
3. Go here and submit your XML sitemap: https://www.google.com/webmasters/tools/sitemap-list?hl=en&siteUrl=http://www.yourdomain.com/
4. Go here and enter the HTML sitemap page in the box, select "web" and click "Fetch": https://www.google.com/webmasters/tools/googlebot-fetch?hl=en&siteUrl=http://www.yourdomain.com/
5. After the page is fetched click on "Submit URL and linked pages to index"
and BOOM! you've created two sitemaps, submitted both and nudged the Googlebot and Bingbot to come crawl your pages and index them.