If it is unlocked it should work with any
GSM provider. Some things, like Data, MMS, and SMS wont work unless you configure the phone for it. You can do this for free on Cingular's website. As far as compatability, get a phone that has
GSM 850 and 1900 and it will work fine in the US. If you will go to Europe get it with 900 and 1800. If you will travel to both, get all frequencies. An excellent place to get a cheaper phone is from ebay. Buy from someone with lots of good feedback.