Virtually anything on your site can potentially affect your conversion rates. Different headlines, pictures, calls-to-action, etc., can all affect your conversion rates positively or negatively.
You have to start with your best guess for each of these but after that, you can be very scientific in discovering the most effective combination. Google Website Optimizer let’s you test any number of combinations of ANYTHING on a web page. You load the various elements into Optimizer and each time someone hits the page, Optimizer serves up a different combination of elements.
You must also place a tracking code in the confirmation page of your conversion page (sometimes called the “thank-you” page). That way Optimizer can measure which combination of elements led to the best conversion rates.
The more elements you test, the more traffic you need before you will have valid results. Google generally recommends about 600-700 visitors for each element that you want to test so if you were testing a headline, intro copy and an image, you would want to wait until you got about 1800-2100 unique visits to the page before drawing any conclusions.