RSS feed and sitemap is not the same thing. sitemap has links to your static pages where rss feed is the post update thta usually lists 10 latest posts. it woul dbe a bad idea to use your feed url as sitemap url. why dont you just setup a sitemap? it doesn't take a whole lot of time. if you need help, then search for a sitemap generator as there are some free sitemap generator or even wordpress plugins that will setup a sitemap for you
You can't use RSS as your sitemap. I suggest you to make sitemap instead, find some plugins and install them and problem solved.
Yes, you can use an RSS feed as a sitemap as explained here: http://www.google.com/support/webmasters/bin/answer.py?answer=156184&hl=en Click Create a Sitemap and then Using an RSS / Atom feed as a Sitemap. "Google accepts RSS (Real Simple Syndication) 2.0 and Atom 1.0 feeds. If you have a blog with an RSS or Atom feed, you submit the feed's URL as a Sitemap. Most blog software creates your feed for you. Note that the feed may only provide information on recent URLs."
As DocuMaker said, you can use RSS feed as a sitemap. And like DocuMaker said, the disadvantage is that it will onyl show recent posts. Sitemaps are intended to be a navigation of that site so that viewers (generally robots like Google's spiders) may much more easily crawl your website. Sitemap plugins, such as for Wordpress, as designed with that in mind. They provide a complete layout of all content posted. If you're using your rss feed for that purpose, you should be fine. I can't definitely say if that will actually affect anything in the long run as long as you have links from crawled pages pointing to your older posts, such as your index page.