To handle the content switching, I recommend you look into AJAX. This means using the javascript XmlHttpRequest object. There are a few popular AJAX function libraries out there to make it easier. I recommend the excellent one from Yahoo. Check out Yahoo's UI Library.
http://developer.yahoo.com/yui/index.html

Specifically, you want the Connection Manager -- this is Yahoo's AJAX class.
http://developer.yahoo.com/yui/connection/index.html

You'll have to play with the examples a bit to get the hang of it, but it is time spent well.

Before most modern browsers supported AJAX, the popular method was IFRAMES--like you are trying to do. You can make that work. All you need is to have your select list or links of RSS feeds change the 'src' attribute of the iframe. You can do this using getElementById, or simply use the normal window.location = "" method. This is not a complete solution, but enough hopefully for you to dig around for the full solution.

As for consuming RSS feeds, you may find my PHP examples helpful---if you use PHP that is. I have an RSS Consumption Example at http://www.troywolf.com/articles