You will need a GSM phone if you want to use in Europe (predominant technology there). Sprint and Verizon use CDMA. That leaves you with options of the
Treo 680 or the upcoming Windows Mobile powered Treo 750. The Treo 750 will offer faster internet than 680 which only supports EDGE. Treo 650 owners have commented that the 680 is faster. It falls in the middle between 650 and 750, speed wise.
-Chris