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I would go to crutchfield and start reading reviews. you can narrow down head units based on features, then find the best rated and read up.
once you've found some you like it's typically a good idea to play with them in person first .. you can't really learn about a head units quirks until you've gotten some hands on experience with them.
I have the Pioneer AVIC-X930BT and I absolutely love it! The AVIC line seems to have some of the best reviews, you should look into those.
Pioneer has already discontinued the 930bt as the 940bt is about to be released. The primary upgrade will be Android phone compatibility. If you have an Android phone check out the SPH-DA100 from Pioneer due out soon. It's $300 less then the new 940bt but it will use the phone for it's nav/traffic and Pandora features. About time they paid attention to something other then the Iphone. No DVD playback if that is important, but I have a feeling there will be lots of new apps that will run on this radio.