Hsdpa vs. GSM vs. UMTS

I have gotten myself totally confused. I attempted to do a search for this subject but just started getting even more confused. I am looking to get an unlocked phone for ATT, I would like it to have 3G. I understand that ATT will be converting all of its 3G networks to the 850mhz band eventually. We currently run 850mhz 3G in my area. So my confusion is when I am looking for a phone and it says GSM 850, I am currently under the impression of that being the edge network, is this correct? From what I can gather, I need to look for a phone that has HSDPA 850 or UMTS 850 if I want to make use of ATT 3G. Am I even close to be correct here? If someone could please explain to me the different aspects of the network types I would be extremely grateful. I am looking to get an Android phone and since there are no native Android phones to ATT, I have to get an unlocked phone. Any help here would be awesome. Sorry if I didnt write my question clearly.

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