From experience, shared & personal, the majority of places which sell the converter boxes are almost completely ignorant.
Best recommendation I know of is referring them to 800-DTV-2009 which is the phone number related to www.dtv2009.com. They state that "We're available to take questions or take coupon requests 24 hours a day, 7 days a week." Let 'em have it!
Also, http://www.dtvboxanswers.com/ gives some reviews on the various boxes. But I recommend www.savingslot.com: they actually list the specifications sheets on boxes and have a variety of boxes available. They were fast, accurate and easy to use.
Regards,