you can get the XM stereo from the dealership too. all you need to do is order it from them. then i dont think that will void the warranty. if you guys have questions, there are yahoo groups that can help you, but some of those people are critical and rude.
www.greenhybrid.com has all kinds of hybrid owners that can help answer questions and they have an online fuel economy database where you can imput data for your own car. Try asking your questions there!