Hi Paul,
You first need to set up your site as an E-commerce site in Analytics (Analytics Settings page > Edit a profile > Click the top right Edit link > Check "Yes, an E-commerce Site").
Then you can add some JavaScript to your "payment successful" page to track the transaction. More info at:
http://code.google.com/apis/analytics/docs/tracking/gaTrackingEcommerce.htmlExactly how you do this depends on the cart/payment system you are using.
You'll then see an Ecommerce item in the left hand menu in the reports. Click this, then click Keywords under Traffic Sources to see which keywords generate the most revenue.
Hope that helps!
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Matt Doyle, Elated
3rd Edition of my jQuery Mobile book out now! Learn to build mobile web apps. Free sample chapter: http://store.elated.com/