GSM in comparison with the UMTS

I got myself in disbelief. I tried to do a search on this topic, but just started to get even more confusing. I'm looking to get a unlocked phone for ATT, I would like to have 3G. I understand that ATT will convert all of their 3G networks in 850 MHz band in the end. We currently run 850MHz 3G in my area. So my confusion when I'm looking for the phone and he says, GSM 850, now I'm under the impression that, as the edge of the network, is this correct? From what I can gather, I need to find a phone that has HSDPA UMTS 850 or 850, if I want to use ATT 3G. I'm even close to correct here? If someone could please explain to me the different aspects of the network type, I would be extremely grateful. I'm looking to get an Android phone, and so there are no native Android phones for ATT, I have to get the phone unlocked. Any help here would be awesome. Sorry if I didn't write my question clearly.