A good way to test and to improve the overall performance of your website is to do A/B testing also known as split testing. Split testing entails running two different versions of a particular page simultaneously for different users and recording how well each page converts.

A more advance version of these tests are known as multivariate testing. A multitude of variables are tested to discover the best combination.

Google offers a tool to do exactly this. It is called Google Website Optimizer. All you need is provide different elements for example different headlines or different product pictures and Google Website Optimizer will serve random combinations of them to your visitors. It also tracks how well each combination converts.

Taken directly from Google: http://www.google.com/support/websiteoptimizer/bin/answer.py?hl=en&answer=55894

“Website Optimizer is a tool that can help you improve the effectiveness of your website in getting a return on your investment. By allowing you to test different versions of your site content, you can determine what will best attract users and lead them to convert on your site.”

This is a free tool so be sure to check it out.